Add command key bindings to macOS text editing and fix selection. (#44130)

This adds support for the command key for text selection/editing on macOS. I had ported the text editing code (in #42879), but forgot to add support for the command key itself. This also adds a test that tests the text editing on multiple platforms instead of just testing Android.

There appears to still be a bug (filed #44135) where we're losing key events sometimes on macOS, leaving some keys "stuck" on, but this PR at least allows the right key combinations to be used.