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.
This updates the keyboard maps to use updated HID codes for game controller buttons (from Chrome), and to encode that GLFW "super" keys are what we call "meta" keys (i.e. Windows key or Command key).
This adds support for game controller buttons. It adds some supplemental USB HID codes that aren't available from the Chromium source code, and maps those on Android to the game pad buttons that Android supports. Other platforms are not supported yet.
This adds a keycode generator that incorporates input from the Chromium and Android source trees, as well as some local tables, to generate static constants for the LogicalKeyboardKey and PhysicalKeyboardKey classes, as well as mappings from each of the platforms we support so far (currently only Android and Fuchsia).
This code generator parses the input files, generates an intermediate data structure (`key_data.json`) that is checked in, and then generates the Dart sources for these classes and some static maps that will also be checked in (but are not included in this PR).
The idea is that these codes don't change often, and so we don't need to generate them on every build, but we would like to be able to update them easily in the future if new data becomes available. If the existing data disappears or becomes unusable, we can maintain the checked-in data structure by hand if necessary, and still be able to generate the code.
This PR only contains the code generator, not the classes themselves. In another follow-on PR, I'll run the generator and check in the output of the generator.
