This adds generation of some C++ keycode maps for various platforms.

This isn't meant for general use: it's so that the Flutter authors can keep the keycodes up to date with the Chromium and Android (and other) sources. It doesn't run continuously, it's only run occasionally (manually) to update the maps, because they don't change very often, and there is often manual work needed to make it function.

In this PR, I added output of various .cc files that write into an engine repo situated next to the flutter repo on disk. This is a starting point for implementing platform-side handling of key events.