Allow `meta` on linux

the keymaps package says "You can use shift, ctrl, alt, meta, option (alt), command (meta), cmd (meta) as modifiers. Note that if you defined a custom shortcut with cmd, command or meta, the same keymaps file won't work on a Windows/Linux machine as this key doesn't have an equivalent."

but, linux does have an equivalent key. super.

i use super extensively in my emacs and vscode keybindings, have for years. would you be open for a PR adding a super alias for meta and making it available on Linux?

i'd imagine ctrlCmd would continue to map to ctrl on linux and meta/super would be an additional option for bindings under Linux and macOS.