Add support for game window embedding on macOS

[jrapf32]

Describe the project you are working on

I'm working on a game that needs object selection, and it's getting tiresome having to switch to the editor, hit the 2d button in the game tab, and then switch back.

Describe the problem or limitation you are having in your project

Currently, "Game embedding is not available on your OS." People use Macs to make games, which could be a problem.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Adding it would help.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

It should work like all other platforms.

If this enhancement will not be used often, can it be worked around with a few lines of script?

If this is not implemented, people can still use Cmd+Tab to switch between windows, but it will be inconvenient.

Is there a reason why this should be core and not an add-on in the asset library?

This functionality is built into the editor and can't be changed by an addon.

[tetrapod00]

The need for this is known, and there are some ideas on how to accomplish it. See comments in the original PR implementing the feature: godotengine/godot#99010 (comment) and godotengine/godot#99010 (comment).