rewrite-url docs: Use languageOptions in stylelint config

[ehoogeveen-medweb]

Now that Stylelint 16.17.0 is out with support for declaration-property-value-no-unknown in languageOptions, the suggested config for postcss-rewrite-url can be generalized. This commit makes the following changes:

• Use languageOptions instead of overriding declaration-property-value-no-unknown rule config
• Make syntax consistent with JSON config: Only use double quotes, no trailing commas

Preview: https://github.com/ehoogeveen-medweb/postcss-plugins/blob/rewrite-url-update-docs/plugins/postcss-rewrite-url/README.md#stylelint

I considered adding a fallback for Stylelint <16.17.0 but given that it's a minor version bump I expect people will update (and if they're looking at postcss-rewrite-url for the first time, they're probably in a good position to update stylelint anyway).

Using the JSON style for the config is a choice I don't feel strongly about - I'm still using it that way in my own projects, but here I just kept it because it seemed consistent with the quoted keys (although the markdown block is technically using language js).

[ehoogeveen-medweb]

Ah, maybe the preceding text could use an update too, as it currently links to and talks about declaration-property-value-no-unknown specifically.

Also, I wonder if it would make sense to do this:

	"types": {
		"rewrite-url()": "rewrite-url( <string> <url-modifier>* )",
		"url": "| <rewrite-url()>"
	}

It might eventually help configure function-no-unknown as well, though I believe this rule doesn't support languageOptions yet.

[romainmenke]

Thank you for this @ehoogeveen-medweb 🙇

I think the current change is fine for now :)
Making the type reusable is indeed better but also requires the reader to know a bit more about the syntax definitions.

[romainmenke]

Thank you. LGTM 👍