eslint-plugin-lit-a11y: flat config support & improvements for d.ts

[what1s1ove]

Hey guys! First of all, thank you for such a cool plugin and all you do!

Since ESLint 9 has been released, are there any plans to add a Flat config and, equally importantly, add the d.ts files of the plugin?

[thepassle]

Yeah, can you create a PR? 🙂