[3.3] Rewrite parser and fix escaped quotes in multiline values

[GrahamCampbell]

Closes #319.

[drzraf]

This commit added symfony/polyfill-ctype to composer.json instead php[ctype].
I can't see the rational of this choice. (And I'd rather before off this extra-dependency)

[GrahamCampbell]

This adds the dependency because we are using the ctype functions.

[GrahamCampbell]

The symfony/polyfill-ctype package has ext-ctype as a recommended package. When you install phpdotenv and you don't have the php extension, composer will actually warn you about this, and recommend you install it.

[drzraf]

... but if one has php[ctype] it installs an unneeded dependency.

[GrahamCampbell]

You can stop this happening in your app with the following trick. In your app's composer.json, do this:

{
    ...
    "replace": {
        "symfony/polyfill-ctype": "*"
    },
    "require": {
        ...
        "ext-ctype": "*",
        ...
        "vlucas/phpdotenv": "^3.3"
    },
    ...
}