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/keymap/context.rs

@@ -25,6 +25,20 @@ impl<'a> TryFrom<&'a str> for KeyContext {
 }
 
 impl KeyContext {
+    /// Initialize a new [`KeyContext`] that contains an `os` key set to either `macos`, `linux`, `windows` or `unknown`.
+    pub fn new_with_defaults() -> Self {
+        let mut context = Self::default();
+        #[cfg(target_os = "macos")]
+        context.set("os", "macos");
+        #[cfg(target_os = "linux")]
+        context.set("os", "linux");
+        #[cfg(target_os = "windows")]
+        context.set("os", "windows");
+        #[cfg(not(any(target_os = "macos", target_os = "linux", target_os = "windows")))]
+        context.set("os", "unknown");
+        context
+    }
+
     /// Parse a key context from a string.
     /// The key context format is very simple:
     /// - either a single identifier, such as `StatusBar`