feat: Add select-recently-used-on-close to the preferences page, b=no-bug, c=no-component

locales/en-US/browser/browser/preferences/zen-preferences.ftl

@@ -53,8 +53,8 @@ category-zen-workspaces =
     .tooltiptext = { pane-zen-tabs-title }
 pane-settings-workspaces-title = Workspaces
 
-zen-tabs-unloader-enabled =
-    .label = Enable Tab Unloader
+zen-tabs-select-recently-used-on-close =
+    .label = When closing a tab, switch to the most recently used tab instead of the next tab
 
 zen-tabs-close-on-back-with-no-history =
     .label = Close tab and switch to its owner tab (or most recently used tab) when going back with no history

src/browser/components/preferences/zenTabsManagement.inc.xhtml

@@ -37,15 +37,10 @@
             class="description-deemphasized"
             data-l10n-id="zen-tabs-cycle-by-attribute-warning"
             hidden="true"/>
+      <checkbox data-l10n-id="zen-tabs-select-recently-used-on-close"
+            preference="zen.tabs.select-recently-used-on-close"/>
 </groupbox>
 
-<hbox id="zenTabsUnloadCategory"
-      class="subcategory"
-      hidden="true"
-      data-category="paneZenTabManagement">
-<html:h1 data-l10n-id="pane-zen-tabs-unloader-title"/>
-</hbox>
-
 <hbox id="zenPinnedTabsManagerCategory"
       class="subcategory"
       hidden="true"