fix: allow additional properties in JSONLanguageOptions

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

While eslint/eslint#19615 (comment) determined that XXXLanguageOptions can have additional properties, and MarkdownLanguageOptions already extends the LanguageOptions type from @eslint/core, the corresponding JSONLanguageOptions does not allow additional properties.

What changes did you make? (Give an overview)

So, I’ve updated JSONLanguageOptions to extend LanguageOptions from @eslint/core and added type test cases to verify that it allows additional properties.

Just a note: JSONLanguageOptions was already being exported from the main entry point, so this PR doesn’t introduce a new feature and only includes a fix.

Related Issues

• Change Request: allow unknown properties in languageOptions eslint#19615 (comment)
• https://github.com/eslint/markdown/blob/main/src/types.ts#L136-L151
• fix: allow additional properties in CSSLanguageOptions css#538

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

I found it while working on eslint/markdown#706.

[DMartens]

Changes LGTM, thanks. Leaving it open for an additional day in case someone else wants to review.