feat(zen-pinned-tab-manager): Add pinned tab management functionality

This commit introduces a new feature: pinned tab management. It allows users to pin tabs and define whether to reset pinned tabs on close.

- Adds a new `ZenPinnedTabManager` component to manage pinned tabs.
- Updates `zen-assets.inc.xhtml` and `zen-assets.jar.inc.mn` to include the new component.
- Modifies the `ZenStartup.mjs` file to initialize the new manager.
- Introduces a new preference `zen.pinned-tab-manager.reset-pinned-tab-on-close-shortcut` to control the behavior of pinned tabs on close.
- Updates the settings UI to reflect the new pinned tab management functionality.
- Refactors related preferences from `zen.tab-unloader.*` to `zen.pinned-tab-manager.*`.

This feature enhances Zen's tab management capabilities by providing users with more control over their pinned tabs.

src/browser/base/content/ZenStartup.mjs

@@ -39,6 +39,7 @@
         gZenUIManager.init();
         gZenVerticalTabsManager.init();
         gZenCompactModeManager.init();
+        gZenPinnedTabManager.init();
 
         function throttle(f, delay) {
           let timer = 0;