Add support for Terminal.app on macOS

We currently expect full RGB support from the hosting terminal. Terminal.app is likely the only popular terminal that doesn't support it, but it's _very_ popular (relatively speaking), so we should add support for it. This requires downsampling the color here:

https://github.com/microsoft/edit/blob/b008570afc0c4c6cef7ba0335d407139a47c8e24/src/framebuffer.rs#L524-L555

And somehow detecting RGB support here:

https://github.com/microsoft/edit/blob/b008570afc0c4c6cef7ba0335d407139a47c8e24/src/bin/edit/main.rs#L499-L595

Detecting RGB support should not use the `COLORTERM` environment variable, because a) I don't like it and b) I'm quite certain there's alternatives to do it. The simplest alternative is to just assume that Terminal.app is the only terminal that doesn't support it (...given that this project doesn't aim to support old terminals).

Start with the framebuffer color handling in src/framebuffer.rs around lines 524–555 and terminal detection in src/bin/edit/main.rs around lines 499–595. Determine how RGB capability can be detected without COLORTERM, then implement Terminal.app support by downsampling colors. Done means Terminal.app renders correctly while terminals with RGB support retain their existing behavior.

Written by the indexing model from the issue text.