Enhance custom fetch tests and docs

enhance custom fetch tests and docs

• Add msw (mock service worker) for testing fetch
  • This doesn't do a lot right now but it will help in the future by allowing us to write tests against this mock server so that we don't need to rely on a hosted Drupal backend.
• Add isomorphic-fetch for testing custom fetch
• Create .eslintrc and extend the root one so lint stops complaining about dependencies not in package.json
• Add vitest to root workspace and globals to the root tsconfig so the docs can build
• Remove explicit vitest imports
• Add typedoc-plugin-mdn-links so relevent docs link directly to MDN references
• Modify customFetch type to allow for the URL type as input