Add generic type aliases to dartdoc

[jcollins-g]

There is now sufficient implementation in top of tree analyzer that we can do this.

[jcollins-g]

Landing this is blocked on #2500, which is in itself blocked on getting build, glob, and dart_style working well with the new analyzer. In a private branch however most of the implementation is complete.

See also dart-lang/sdk#44960.

[parlough]

@jcollins-g If you want to open a draft PR with the in progress changes, I'd be happy to take a preliminary look!

[jcollins-g]

I will be swinging back around to this feature shortly.