Add "Merge all windows" to command palette

[jjt]

The Window menu on MacOS has "Merge All Windows". I'm lazy efficient so I want to use the command palette to merge all windows instead of having to use the mouse or cmd+shift+/ to access the Help > Search menu.

[mitchellh]

This is a native macOS feature. We'd need to expose this as a keybinding and find the appropriate API to call to do this programmatically in order to do this.