Do not expose default slot let bindings to named slots

[tanhauhau]

BREAKING CHANGE

Default slot let bindings are not available on named slots anymore and vice versa:

<script>
	import Nested from './Nested.svelte';
</script>

<Nested let:count>
	<p>
		count in default slot - is available: {count}
	</p>
	<p slot="bar">
		count in bar slot - is not available: {count}
	</p>
</Nested>

This makes slot bindings more consistent as the behavior is undefined when for example the default slot is from a list and the named slot is not.

To fix this, make the bindings you need available on the named slot (or default slot) and bind to them there.

<script>
	import Nested from './Nested.svelte';
</script>

<Nested let:count>
	<p>
		count in default slot - is available: {count}
	</p>
--	<p slot="bar">
++	<p slot="bar" let:count>
--		count in bar slot - is not available: {count}
++		count in bar slot - is now available: {count}
	</p>
</Nested>

PR description

Fixes #5865
Fixes #5684
Fixes #7245
Fixes #7709

@Rich-Harris this takes an opposite approach of #2419 from fixing #2320
where defining the let:value on Component, <Component let:value> will only apply to the default slot.

To make it clear,

• it will not create a new scope on the <Component> level
  • using value in named slot will have "missing-declaration" warning
• it will emit warnings, discouraging using let: on the <Component>
  • only if there's other named slots within <Component >

---

#2419 exposes the default slot data to named slot:

<!-- Component.svelte -->
<slot data="123" />
<slot name="foo" />

<!-- App.svelte -->
<Component let:data>
  <svelte:fragment slot="foo">
    {data}
  </svelte:fragment>
</Component>

This assumes that the default slot will always be there whenever foo slot exists, and there will be 1 default slot.
This immediately turns complicated and does not make sense if there's multiple of default <slot />

{#each [1,2,3] as item}
  <slot data={item} />
{/each}
<slot name="foo" />

This MR fixes #2320 with a separate direction, where specifying let:data in the <Component> attribute will be solely for default slot and will not be available to the named slot.

Before submitting the PR, please make sure you do the following

• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[dummdidumm]

If we decide to go with it we can only do so for v4, as it's a breaking change

[vercel]

@tanhauhau is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[dummdidumm]

Looks good mostly, left a few comments