This PR includes two relevant changes:
- Platform binds (super, windows, cmd) will now parse on all platforms,
regardless of which one is being used. While very counter-intuitive
(this means that `cmd-d` will actually be triggered by `win-d` on
windows) this makes it possible to reuse keymap files across platforms
easily
- There is now a KeyContext `os == linux`, `os == macos` or `os ==
windows` available in keymaps. This allows users to specify certain
blocks of keybinds only for one OS, allowing you to minimize the amount
of keymappings that you have to re-configure for each platform.

Release Notes:

- Added `os` KeyContext, set to either `linux`, `macos` or `windows`
- Fixed keymap parsing errors when `cmd` was used on linux, `super` was
used on mac, etc.