The following changes are made by this PR:

 * Translation bundles now implement MaterialLocalizations directly,
   and are public so that they can be directly extended.

 * The list of supported languages is now a generated constant.

 * The icuShortTimePattern/TimeOfDayFormat values are now pre-parsed.

 * Various other changes for consistency with the style guide and the
   rest of the codebase, e.g. the class names don't use `_`, the
   `path` library is imported as such, more dartdocs, fewer `//
   ignore`s, validation using exceptions.

This reduces our technical debt benchmark.