fix: enforce strict syntax for @charset in no-invalid-at-rules

[thecalamiity]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

To enforce strict syntax requirements for the @charset rule. This ensures that only valid @charset declarations are accepted.

What changes did you make? (Give an overview)

• Updated the no-invalid-at-rules rule to strictly validate @charset syntax. Now enforces:
  • The encoding must be enclosed in double quotes.
  • There must be exactly one space after @charset.
  • The rule must be immediately terminated with a semicolon.
• Added tests to cover valid and invalid @charset usages.
• Updated the documentation to note the strict requirements for @charset.

Related Issues

Fixes #185

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

[Copilot]

Pull Request Overview

This PR enhances the no-invalid-at-rules rule to strictly validate @charset declarations, ensuring they use double quotes, exactly one space, and terminate with a semicolon.

• Introduced charsetPattern and validateCharsetRule to enforce syntax in the rule implementation
• Added test cases covering valid and invalid @charset variations
• Updated documentation to detail the strict requirements and examples

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/rules/no-invalid-at-rules.test.js	Added valid/invalid test cases for @charset
src/rules/no-invalid-at-rules.js	Implemented validateCharsetRule and regex check
docs/rules/no-invalid-at-rules.md	Added note and examples for strict @charset

Comments suppressed due to low confidence (1)

docs/rules/no-invalid-at-rules.md:75

• [nitpick] Describing @charset as "not an at-rule" is misleading; consider rephrasing to clarify that it is a special at-rule requiring placement as the very first statement in a stylesheet.

Note on `@charset`: Although it begins with an `@` symbol, it is not an at-rule. It is a specific byte sequence of the following form:

[Copilot]

[nitpick] Currently missing semicolon errors are reported as invalidPrelude; consider introducing a distinct messageId (e.g., missingSemicolon) to clearly differentiate terminator errors from prelude syntax issues.

Suggested change

	context.report({
	loc: prelude.loc,
	messageId: "invalidPrelude",
	data: {
	name,
	prelude: preludeText,
	expected: "<string>",
	const isMissingSemicolon = !nodeText.endsWith(";");
	context.report({
	loc: prelude.loc,
	messageId: isMissingSemicolon ? "missingSemicolon" : "invalidPrelude",
	data: {
	name,
	prelude: preludeText,
	expected: isMissingSemicolon ? "; at the end" : "<string>",

[nzakas]

Overall, this looks really good. What do you think about adding autofix to this rule? It seems like it should possible to just reformat the @charset so that it's correct.