Chore: add JSDoc types for RuleTester test cases

[golopot]

What is the purpose of this pull request? (put an "X" next to item)

[x] Other, please explain:

Improve JSDoc typings.

What changes did you make? (Give an overview)
Add type for valid and invalid RuleTester test cases. This addition enables editor autocomplete/suggestions.

The typings are copied from @types/eslint.

Is there anything you'd like reviewers to focus on?
No

[kaicataldo]

Sorry for the delay! This generally LGTM, but I'd love to have others who might have opinions around where the best place for these type definitions is.

[mysticatea]

Thank you for your contribution and I'm sorry for the late responses.

The adding is good to me. But, as similar to other files, it's better to group the type definitions at the top of the file. And I suggested some right types.

[kaicataldo]

@golopot Friendly ping

[golopot]

Ready for review.

[mysticatea]

LGTM. Thank you!

[platinumazure]

This generally looks good to me, but I'll ask one question: Is there a way for InvalidTestCase to require either a message or messageId, but preferably disallow both? Thanks!

[golopot]

Is there a way for InvalidTestCase to require either a message or messageId, but preferably disallow both?

There is a SO question with a well accepted answer https://stackoverflow.com/questions/37688318/typescript-interface-possible-to-make-one-or-the-other-properties-required. But when I try it I found that the answer actually does not work, due the the limitation of TypeScript, see https://stackoverflow.com/questions/59462318/why-does-typescript-allow-a-1-b-2-to-be-assigned-to-type-a-any-b . So, ...no.

[mdjermanovic]

LGTM, thanks for this useful addition!

We can add suggestions later.

[kaicataldo]

Thanks for contributing to ESLint!