First pass at making a linux keymap (#8082)

Undoubtedly not perfect, but this should be something we can work off
of.

Note that matching keybindings with ctrl in them is currently broken on
linux (or at least x11). This keymap might just manage to be less useful
than using the macos one on linux until that is fixed... the proximate
cause of this is that the `key` field of the `Keystroke` struct looks
like `"\u{e}"` instead of `"n"` when `ctrl-n` is pressed.

Release Notes:

- N/A

crates/vim/src/test/vim_test_context.rs

@@ -54,7 +54,7 @@ impl VimTestContext {
             cx.update_global(|store: &mut SettingsStore, cx| {
                 store.update_user_settings::<VimModeSetting>(cx, |s| *s = Some(enabled));
             });
-            settings::KeymapFile::load_asset("keymaps/default.json", cx).unwrap();
+            settings::KeymapFile::load_asset("keymaps/default-macos.json", cx).unwrap();
             if enabled {
                 settings::KeymapFile::load_asset("keymaps/vim.json", cx).unwrap();
             }