fix: Ensure languageOptions.customSyntax is serializable

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Ensures that languageOptions.customSyntax is serializable by replacing all functions with true. CSSTree allows functions in its config and we need to account for that to allow things like caching in ESLint.

What changes did you make? (Give an overview)

• Added CSSLanguage#normalizeLanguageOptions() that adds a toJSON method when the customSyntax option is present
• Updated the CSSLanguage tests to validate the new functionality works.
• Updated the plugin test to ensure we get the expected results when run through ESLint.

Related Issues

fixes #211

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

[Copilot]

Pull Request Overview

This PR addresses a serialization issue with ESLint configuration by ensuring that languageOptions.customSyntax containing functions can be properly serialized to JSON. This is necessary for ESLint features like configuration caching.

• Added normalizeLanguageOptions() method to handle serialization of customSyntax configurations
• Enhanced test coverage for both unit and integration scenarios
• Updated JSDoc documentation for consistency

Reviewed Changes

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

File	Description
src/languages/css-language.js	Implements serialization logic with normalizeLanguageOptions() method and helper function
tests/languages/css-language.test.js	Unit tests for the new normalization functionality
tests/plugin/eslint.test.js	Integration tests validating serialization behavior through ESLint

[Copilot]

[nitpick] The method modifies the input object by adding a toJSON property. Consider returning a new object instead of mutating the input to avoid side effects and make the API more predictable.

[fasttime]

Would it make sense to set the toJSON method on a shallow clone of the argument (results) so that the original object isn't modified?

Suggested change

	Object.defineProperty(languageOptions, "toJSON", {
	value() {
	// Shallow copy
	const result = { ...languageOptions };
	// Shallow copy
	const result = { ...languageOptions };
	Object.defineProperty(result, "toJSON", {
	value() {

We'd need to change the return value of the function as well and update some unit tests.

[fasttime]

It looks like this function also returns arrays and non-object values depending on the type of the argument?