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

首先检查现有的 pattern mappings 和 KeyCode 构造函数，然后比较所提议的独立模块、Backpack 接口和 phantom class HasKeyCodes 浏览器方案。在实现之前，确定需要哪种浏览器选择设计，并将 done 定义为：可以选择特定于浏览器的 mappings，且不会遗留未解决的部分 patterns。

由索引模型根据 Issue 内容生成。