[ValentinYoushkevich]

This is the follow-up promised in #3236 for the internal lint rules and the docs build shims. The three files in eslint-internal-rules/ are now .ts with ESM exports, and eslint.config.mjs imports them directly, so Node runs them through type stripping exactly like the rules in lib/rules, which the config already loads through tools/lib/rules.js. The folder is added to tsconfig.json, so npm run tsc checks it too. In the docs, shim/empty.mjs and src/process-shim.mjs became .ts; they are only consumed by Vite and esbuild, and process-shim.ts uses the same @ts-expect-error -- Browser comments for window as the theme does. Behaviour is unchanged. docs/.vitepress/public/service-worker.js and the integration fixtures' configs stay JavaScript on purpose. The tools/ migration is in #3237.

Part of #2777

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