macOS: support selection pasteboard

[mitchellh]

Discussed in #2670

^{Originally posted by lilyball November 13, 2024}
One of my favorite Terminal.app features is it has a menuitem ⇧⌘V for Paste Selection. What this does is whatever the last text I selected in Terminal.app was, it pastes that. So this is kind of like copy-on-select behavior except it stores it in a separate pasteboard and only pasted when I use the Paste Selection action. This can be used across windows too, and it's also persisted across relaunch. I'd really love to see Ghostty pick up the same feature.

Implementation note: my initial guess here is Terminal.app uses +[NSPasteboard pasteboardWithName:] to create a private pasteboard for this purpose, which would also explain how it persists it (since I don't see it writing the string to preferences). I don't know what name it's using for this. If Ghostty does this as a macOS-only feature then it could perhaps do the same thing.

I believe we should be able to implement this already since we support a selection pasteboard on Linux. I think this is probably almost purely a Swift thing, maybe some embedded apprt work but not much.