Add a shared key-code lookup for macOS and Windows

Maps the SDK's shortcut key names to macOS and Windows key codes in one place. Keys resolve through the current keyboard layout (letters and punctuation on macOS, punctuation on Windows, where letter codes already follow the layout), so a chord like Cmd+V still works on Dvorak and AZERTY. This is the base of a stack: the global shortcuts and keyboard input PRs both use it.