Make keymaps reusable across platforms (#10811)

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.

crates/gpui/src/platform/keystroke.rs

@@ -74,12 +74,7 @@ impl Keystroke {
                 "alt" => alt = true,
                 "shift" => shift = true,
                 "fn" => function = true,
-                #[cfg(target_os = "macos")]
-                "cmd" => platform = true,
-                #[cfg(target_os = "linux")]
-                "super" => platform = true,
-                #[cfg(target_os = "windows")]
-                "win" => platform = true,
+                "cmd" | "super" | "win" => platform = true,
                 _ => {
                     if let Some(next) = components.peek() {
                         if next.is_empty() && source.ends_with('-') {