Add Xcode build script for macOS target

[jonahwilliams]

Description

Removes indirection by moving the macOS build script to flutter_tools. This still uses the FDE asset syncing. This uses almost the same scheme/flag passing setup as the existing iOS build.

Currently the track-widget-creation flag is still broken though, it seems like some part of the process is using the regular .dill instead of the .track.dill - still investigating.

Fixed

See also: google/flutter-desktop-embedding#336

Related Issues

Fixes #31274

#30707
#30706

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[stuartmorgan]

I haven't had a chance to try this out locally yet (will do that tomorrow, and then re-review) but looks generally good so far.