v5: Modal focus changes

Skipping some previous v4-based PRs given our move to regular JS has already landed in master.

• #25277 (closed): Modal tabbing and shift+tabbing behavior
• #27646: Allow shift-tab focusing for modals
• #25651: Modal input autofocus attr & keyboard nav
• #21661 (closed): shift+tab without focused element within modal
• #27241 (closed): SHIFT + TAB should cycle backwards

/cc @Johann-S