AppHosting startCommand & framework port limitation

https://github.com/firebase/firebase-tools/blob/729dbcfca4a260543d30cb08781717c027a9c369/src/emulator/apphosting/serve.ts#L109-L123

We just upgraded from 14.10.x and the emulator stopped working.

The logic prevents that we use `--port` in the command, which is too restrictive, specially taking into account that the option is called `startCommand` and allows any arbitrary command.
Further on that logic, it adds the port only if the command contains `ng serve`, which is Angular only, and it makes no sense, since it can easily be a call to a custom script.

In my opinion, this isn't a good approach, unless you want to reduce flexibility, which in that case I wouldn't even provide a custom `startCommand` at all.

I recommend that either
- you remove this limitation and document the user should pass the port manually in the start command (as we were doing already in our team)
- or you pass an arbitrary `-- --port $port` no matter the command that is set up by the user and document that option too

This change was introduced in `14.17.0`, but I believe this should have been flagged as a breaking change. Unfortunately, nothing was warned or documented either.

Thank you.