chore: upgrade @eslint/core and @eslint/plugin-kit

[fasttime]

This PR promotes @eslint/core to a runtime dependency and upgrades @eslint/core and @eslint/plugin-kit to the latest versions.

This change ensures that @eslint/markdown will be used with a compatible version of @eslint/core in dependent packages even if the core types are changed in a breaking way in the future. This will help avoiding bugs like #313 where the removal of a type from @eslint/[email protected] to @eslint/[email protected] was causing the TS build to fail. Since @eslint/core was defined in the devDependencies, npm would not check its version range when @eslint/markdown was itself installed as a dependency.

I also made a change to the tsconfig.json in the type tests because I noticed that some files in the dist directory were not being checked.

[mdjermanovic]

LGTM, thanks!