This adds support for the fn key on macOS. It adds it to the key mappings as a supplemental mapping that overwrites the one from the Chrome headers, since the chrome headers have a TODO, but no implementation of the key.

Also, ignore the fn key entirely on macOS. This is because On macOS laptop keyboards, the fn key is used to generate home/end and f1-f12, but it ALSO generates a separate down/up event for the fn key itself. Other platforms hide the fn key, and just produce the key that it is combined with, so to keep it possible to write cross platform code that looks at which keys are pressed, the fn key is ignored.