no-bug: Add keyboard shortcuts to move tabs between workspaces

[kristof-nyr]

Summary

Adds 12 new keyboard shortcuts to move the active (or multiselected) tabs to a specific workspace or to the next/previous workspace.

Addresses community requests in:

• Keyboard shortcuts to move tabs to workspaces #4805
• add shortcuts to move selected tabs to backward/forward workspace #4363
• When using "change tab(s) to workspace" to send tabs to another workspace it should not switch the workspace #1058

What's included

• Commands: 12 new <command> elements in zen-commands.inc.xhtml
• Move logic: shortcutMoveTabTo(index) and moveActiveTabShortcut(offset)
  in ZenSpaceManager.mjs; empty tabs (zen-empty-tab) are filtered out to
  prevent moving placeholder tabs
• Feedback: Toast notification ("Moved tab to [name]" / "Moved N tabs to
  [name]") using existing gZenUIManager.showToast; the toast container's ARIA
  live region provides screenreader announcements automatically
• Shortcuts: Registered in ZenKeyboardShortcuts.mjs with migration to
  version 19; extracted shared createMoveTabShortcuts() factory so fresh-install
  and migration paths cannot diverge
• Defaults: Ctrl+Shift+1..0 on macOS (mirrors Ctrl+1..0 for switching);
  unbound on other platforms
• Tests: 10 mochitest tasks covering single move, multiselect, wrap/no-wrap,
  same-workspace guard, empty-tab fallback, and toast verification

Design decisions

• User stays in the current workspace after moving a tab (matches Discussion
  When using "change tab(s) to workspace" to send tabs to another workspace it should not switch the workspace #1058 consensus)
• Shortcuts unbound by default on non-macOS (mirrors existing workspace-switch
  pattern)
• Out-of-bounds and same-workspace moves are silent no-ops
• Empty/placeholder tabs are skipped (no-op if workspace only has an empty tab)

Behavioral note for reviewers

In zen-sets.js, the default case was reordered: workspace-switch and
move-tab commands are now checked before gZenGlanceManager.handleMainCommandSet().
Previously the glance manager saw all default-case commands including workspace
switches. The new order only passes truly unrecognized commands to the glance
manager. This is more correct (workspace commands aren't glance-related) but
reviewers should confirm the glance manager doesn't depend on seeing those events.