feat: Add types to package

[nzakas]

Builds on #178 to include types in the package:

• Add type definitions (copied from @types/eslint__eslintrc)
• Add type tests
• Run type tests in CI

This eliminates the need for @types/eslint__eslintrc.

[fasttime]

LGTM, thanks!

[fasttime]

I'll leave this open until today's release in case someone else would like to review.

[fasttime]

Oh, actually the path of the .d.ts file in package.json is wrong. I was wondering why VSCode wasn't finding the new types locally.

[nzakas]

Good catch!

[fasttime]

Thanks. Going to merge now.