…` symbols in `.phpstub` files

Also:

 * added PHP 8.2 stubs
 * refined types to make impossible scenarios more clear (like `ReflectionIntersectionType#allowsNull()`)

This is a first attempt at refining these types: the structure of these stubs is quite confusing to me,
so I don't know if this approach is correct, and if the stubs are merged together, or if entire symbols
need to be completely re-declared for each PHP version.

…-string`

Because @weirdan is a party pooper (they poop at the parties)

Ref: https://www.youtube.com/watch?v=gjwofYhUJEM
Ref: vimeo#8722 (comment)

As per @weirdan's feedback, we can prevent
the usage of `object` instances that
implement `__invoke()`, as well as `array`
callables, by declaring the ctor argument of
`ReflectionFunction` to be either a real `Closure`,
or a `callable-string`.

While this may not be 100% of scenarios, it is a
healthy way to identify errors in userland.

Ref: vimeo#8722 (comment)

These templates were leading to false positives: assuming
an `object` is given as input, the inferred return
type would always have been `true`, which is obviously
not valid.

Removing them is the healthier alternative, for now.

Ref: vimeo#8722 (comment)

… 8.1 and PHP 8.2

* in PHP 8.0, `ReflectionUnionType` is composed on `ReflectionNamedType`s
* in PHP 8.1, `ReflectionIntersectionType` is composed of `ReflectionNamedType`s
* in PHP 8.2, `ReflectionUnionType` is composed of `ReflectionIntersectionType|ReflectionNamedType`s

Slight variations for each PHP version.

As per local testing, this doesn't work yet.

## Local testing setup:
I did some digging to make sure that the stubs work as expected.

Here's what I did to validate this patch locally (since I don't think it can really be fully automated)

## Create a dummy file to verify used symbols

```php
<?php

namespace Testing;

/** @return \ReflectionClass<\stdClass> */
function getAClass(): \ReflectionClass { throw new \Exception('irrelevant'); }
function getAnUnionType(): \ReflectionUnionType { throw new \Exception('irrelevant'); }
function getAnIntersectionType(): \ReflectionIntersectionType { throw new \Exception('irrelevant'); }

// verifying that `getName()` is stubbed in all versions: this should always be a `class-string<\stdClass>`
$name = getAClass()->getName();
// union types should appear starting with PHP 8.0. Starting with PHP 8.2, they allow for intersections.
$unionTypes = getAnUnionType()->getTypes();
// intersection types should appear starting with PHP 8.1
$intersectionTypes = getAnIntersectionType()->getTypes();

$results = [$name, $unionTypes, $intersectionTypes];

/** @psalm-trace $results */ // tracing this will show us the differences between versions
return $results;
```

## Run the script against various `vimeo/psalm` versions

```sh
docker run --rm -ti -v $(pwd):/app -w /app php:7.4 ./psalm --php-version=7.4 --no-cache reflection-test.php | grep Trace

docker run --rm -ti -v $(pwd):/app -w /app php:8.0 ./psalm --php-version=8.0 --no-cache reflection-test.php | grep Trace

docker run --rm -ti -v $(pwd):/app -w /app php:8.1 ./psalm --php-version=8.1 --no-cache reflection-test.php | grep Trace

docker run --rm -ti -v $(pwd):/app -w /app php:8.2.0RC7-cli ./psalm --php-version=8.2 --no-cache reflection-test.php | grep Trace

```

## Evaluate output

```
❯ docker run --rm -ti -v $(pwd):/app -w /app php:7.4 ./psalm --php-version=7.4 --no-cache reflection-test.php | grep Trace
ERROR: Trace - reflection-test.php:20:1 - $results: list{class-string<stdClass>, mixed, mixed} (see https://psalm.dev/224)

❯ docker run --rm -ti -v $(pwd):/app -w /app php:8.0 ./psalm --php-version=8.0 --no-cache reflection-test.php | grep Trace
ERROR: Trace - reflection-test.php:20:1 - $results: list{class-string<stdClass>, non-empty-list<ReflectionNamedType>, mixed} (see https://psalm.dev/224)

❯ docker run --rm -ti -v $(pwd):/app -w /app php:8.1 ./psalm --php-version=8.1 --no-cache reflection-test.php | grep Trace
ERROR: Trace - reflection-test.php:20:1 - $results: list{class-string<stdClass>, non-empty-list<ReflectionNamedType>, non-empty-list<ReflectionNamedType>} (see https://psalm.dev/224)

psalm on  feature/vimeo#8720-improve-types-and-purity-for-reflection-symbols [!?] via 🐘 v8.1.13 via ❄️  impure (nix-shell) took 4s
❯ docker run --rm -ti -v $(pwd):/app -w /app php:8.2.0RC7-cli ./psalm --php-version=8.2 --no-cache reflection-test.php | grep Trace
ERROR: Trace - reflection-test.php:20:1 - $results: list{class-string<stdClass>, non-empty-list<ReflectionNamedType>, non-empty-list<ReflectionNamedType>} (see https://psalm.dev/224)
```

…rom PHP 8.1 onwards

This will highlight unused code.

Ref: php/php-src#5412
Ref: https://wiki.php.net/rfc/make-reflection-setaccessible-no-op
Ref: php/php-src#5411

Example https://3v4l.org/PNeeZ

```php
<?php

class Foo {
    private $bar = 'baz';
    private function taz() { return 'waz'; }
}

//var_dump((new ReflectionProperty(Foo::class, 'bar'))->getValue(new Foo));
//var_dump((new ReflectionMethod(Foo::class, 'taz'))->invoke(new Foo));
```

Produces (starting from PHP 8.1):

```
string(3) "baz"
string(3) "waz"
```

…come in a `list`

Before this change, preloaded stubs would only be loaded when running on a PHP version lower than
the one that is in the stubs.

With this change, the analysis PHP version (defined via config, input parameter, or inferred from
the runtime) becomes authoritative.

Since the PHP-version-specific stubs are not just polyfills, but actually type refinements on top
of the PHP core symbols at hand, this change always loads them, so that it is possible to get more
precise type inference downstream.

This will likely lead to downstream breakages, because the stubs do indeed provide better type
resolution, but indeed formalizes the idea that these stubs will provide better value for finding
problems in analyzed code.

…ly when visiting stub files

While `visitPreloadedStubFiles` seemed to work at first, it led to overriding symbols declared by
PHP itself too eagerly.

By only loading PHP-version-specific stubs in `visitStubFiles`, we ensure that the reflection-declared
symbols take priority, and that our stubs overlay on top of that, without actually replacing the
symbol entirely, but rather merging with its definition.

This fixes current test failures too, and works with the code examples
from vimeo#8722 (comment)