A middleware can work as a proxy intercepting requests and then performing a `fetch` to the destination adding headers to the request / response as a "man in the middle". When using `fetch` from a middleware we are not in the context of a browser so we can't really use relative URLs, they must be always absolute.
Now consider the previous case when middleware is running in *server mode*. Typically in order to know the host where we are fetching we can use the `request.nextUrl` which is given to the middleware but in this case the invoker (which is next-server) has no context of the hostname, nor the port. To solve this use case we must make the invoker of the middleware aware of the origin hostname and port.
This PR:
- Introduces `hostname` and `port` as options for `NextServer`.
- Refactors types in `NextServer` and `NextDevServer` moving type only imports to the top of the file.
- Refactors `startServer` to do a best guess on the `hostname` and `port`, passing them down.
- Exposes `.port` and `.hostname` to be retrieved from the `app`.
In an upcoming PR we will pass the host guess to the middleware to solve the relative URL issue.
- Enables excludeDefaultMomentLocales by default
- Adds distDir cleaning (See RFC #6009)
- Adds support for `PORT`
- Removes `router.events` from the server-side router as it should not be used server-side (long-standing todo that is potentially breaking). Note that it's still available as `Router.events` (import Router from 'next/router') and with `useRouter` in `useEffect`. Using it with `useEffect` is the correct way and I've updated the upgrading guide to reflect that
- Added webpack 5 to the upgrading guide
- Removed `Head.rewind` as it's been a no-op since Next.js 9.5 and can now be safely removed from user code
Fixes#11408Fixes#10338Fixes#5554
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
## Feature
- [x] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [x] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
## Documentation / Examples
- [ ] Make sure the linting passes
* Run prettier over packages/**/*.js
* Run prettier over packages/**/*.ts
* Run prettier over examples
* Remove tslint
* Run prettier over examples
* Run prettier over all markdown files
* Run prettier over json files
