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/project_panel/src/project_panel.rs

@@ -1698,7 +1698,7 @@ impl ProjectPanel {
     }
 
     fn dispatch_context(&self, cx: &ViewContext<Self>) -> KeyContext {
-        let mut dispatch_context = KeyContext::default();
+        let mut dispatch_context = KeyContext::new_with_defaults();
         dispatch_context.add("ProjectPanel");
         dispatch_context.add("menu");