feat: Allow custom syntax

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Adds support for custom syntax to the parser.

What changes did you make? (Give an overview)

• Added languageOptions.customSyntax to CSSLanguage
• Updated rules to use custom syntax
• Updated tests for CSSLanguage and rules
• Updated README
• Added /syntax entrypoint for common custom syntax like Tailwind

Related Issues

fixes #37

Is there anything you'd like reviewers to focus on?

There is a limitation in CSSTree that prevents custom functions like Tailwind's theme() to work properly, so I left that out:
csstree/csstree#292

When that issue is resolved, we can revisit.

[fasttime]

There's a merge conflict.

[mdjermanovic]

• Updated tests for CSSLanguage

There are currently no updates in tests for CSSLanguage, maybe some commits are missing?

[nzakas]

I didn't include any additional tests for CSSLanguage because the new functionality can only really be tested by running the rule to validate syntax.

I did add a test that shows the new option throws when invalid.

[mdjermanovic]

LGTM, thanks!