feat: add tab group support (skip collapsed groups, zc/zj/zk navigation)

[tusharmurali]

Description

Fixes #4693
Related #3937, #4639

This PR adds tab group support to Vimium:

1. Skip collapsed tab groups when navigating (J/K/gt/gT/g0/g$)

Chrome's native Ctrl+Tab skips collapsed tab groups, but Vimium's J/K currently navigates into them. This aligns Vimium's behavior with Chrome's native UX.

2. New commands for tab group navigation (Vim fold-style keybindings)

Key	Command	Description
zc	collapseTabGroup	Collapse current tab's group
zj	previousTabGroup	Switch to previous tab group
zk	nextTabGroup	Switch to next tab group

These follow Vim's fold navigation conventions (z prefix, j/k for direction).

Notes:

• Adds tabGroups permission (no new user-facing permission warnings)
• Firefox: gracefully no-ops (Firefox 140+ will add tab group API support)
• Users can still access tabs in collapsed groups via T (Vomnibar tab search)