Misc. Manage Library changes (including temp. hiding "Add folder" behind developer mode flag)

• "Add" dropdown trigger button added to Manage Library panel
• "New folder" button:
  • moved to dropdown at top of Manage Library panel
  • hidden behind devMode flag.
• "New code component" moved from Library into dropdown at top of Manage Library panel
• 0 shouldn't show when there are no folders or items in a folder
• Completely refactored PatternList, ComponentList, CodeComponentList to all use a centralised "LibraryItemList"
• 🔥 Working Search/filter added to both Library and Manage Library
  • Search persists across the "Components" "Patterns" and "Code" tabs
  • Info panel is shown when nothing in the current tab matches the search term.
• Clicking a component no longer immediately inserts it
  • Components in Library now have contextual menu option "Insert"
  • Cypress and Playwright tests updated to insert components using a centralised "insertComponent" helper function that uses the contextual menu

From the D.o. issue Bonus issue 2 is not addressed by this MR. I think it should be tackled in a follow up or as part of https://www.drupal.org/project/canvas/issues/3541364

Dropping this here as proof 😅

Closes #3545710