macOS: allow pasting with command instead of control

Problem

On Windows and Linux, copy/pasting works with control, and the engine is designed to support that. It doesn't currently support command, which macOS users tend to be used to.

Solutions

Allow command-V to paste, command-C to copy, and command-X to cut on macOS.

Alternatives

Make it clear that only control can be used?

Additional context

I'm opening this as a feature request because I'm not sure it counts as a bug (nothing's broken, per se). I can help test the PR that fixes this issue.