feat(experimental-utils): add only property to RuleTester types

[G-Rath]

Was added in latest eslint: eslint/eslint#14677 🎉

(cc @SimenB in case you missed it - I'm so glad we finally have this 😍)

[typescript-eslint]

Thanks for the PR, @G-Rath!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[nx-cloud]

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	nx run-many --target=typecheck --all --parallel

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[bradzacher]

thanks!