Follow up for https://github.com/vercel/next.js/pull/40415
Remove internal next client api determination, fully relying on `'client'` directive.
Change `.client.js` extension to `.js ` in tests, remove legacy / unused test files
## Feature
Change server components convention from using `.server.js` / `.client.js` file extension to determine it's a server or client component to using `'client'` js literal as a directive for determine client components boundary.
React RFC: https://github.com/reactjs/rfcs/pull/189
New behavior doesn't consume `.server.js` as server components any more, if you're enabling `serverComponents` flag, every `page.js` in app dir will become server components by default. If you adding a `'client'` directive to the page, then that page will become a client component. This rule also applies to the normal js components, client components will require a `'client'` directive to indicate its identity, instead of having a `.client.js` extension.
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
Co-authored-by: Shu Ding <3676859+shuding@users.noreply.github.com>
Jiachi Liu