feat: Make it possible to only install Shorebird and get Flutter + Dart

[eseidel]

I doubt we need to do this soon, but eventually we will want to find a different way to hook our shorebird engine into Flutter.

It would allow use of the flutter from shorebird for buiding release builds w/o shorebird (e.g a --no-codepush variant). This would allow users to just use the vended copy of Flutter and not have to install their own.

It also could pave the way for us to provide more build options (not just codepush) since it's likely that we will want to have different codepush build methods once we have an interpreter mode for iOS (that we might also want to offer on Android).

[eseidel]

https://docs.google.com/document/d/1z_4Z5JMTbk5c4FpayVAb-PTz2wLP-aSCirbieavT3Ws/edit?pli=1 might be relevant (although it looks dead).

[eseidel]

This blocks our ability to be the way you install Flutter.

[eseidel]

I think we could start this by just pulling our own dart (same way that flutter does), even if we don't make it available to others. That removes our dependency on what version of dart you have in your path.

[eseidel]

We would have to do a lot to get here. I'm not sure this is worth keeping open as a bug for now.