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

Start by reviewing the existing pattern mappings and the KeyCode constructor, then compare the proposed separate modules, Backpack interface, and phantom class HasKeyCodes browser approaches. Before implementation, establish which browser-selection design is wanted and define done as browser-specific mappings being selectable without leaving partial patterns unresolved.

Rédigé par le modèle d'indexation à partir du texte de l'issue.