Provide browser-specific mapping

Currently some patterns are partial because different browsers have different mappings. We'd like to include a way to choose a browser.

- The simplest way is to put browser specific mappings in separate modules
- This could be improved by interfacing with backpack
- A better way is to make the patterns members of a phantom class `HasKeyCodes browser`. The disadvantage is always needing to feed in the browser type in the type declaration. We could include the phantom type in the `KeyCode` constructor to choose the instance more easily