Refactor ZenStartup.mjs and zen-settings.js

- Refactor ZenStartup.mjs to use gZenKeyboardShortcutsManager instead of gZenKeyboardShortcuts.
- Refactor zen-settings.js to use async/await for _resetCKS and _handleKeyDown functions.

src/browser/base/content/ZenStartup.mjs

@@ -45,7 +45,7 @@
         gZenUIManager.init();
         gZenVerticalTabsManager.init();
         gZenCompactModeManager.init();
-        gZenKeyboardShortcuts.init();
+        gZenKeyboardShortcutsManager.init();
 
         function throttle(f, delay) {
           let timer = 0;