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.