Add a 'switch' command for changing core and contrib module branches.

[amateescu]

Fixes #23.

• New host command: ddev switch core 11.x runs git switch + ddev composer update; ddev switch token 2.0.x switches the module's branch and updates its Composer constraint in one step.
• Runs ddev mutagen sync after the branch switch so the container sees the new files before Composer runs (no-op without Mutagen)
• Git worktree checkouts (.git file instead of directory) are accepted everywhere: the module commands, the core path of switch, and the composer plugin's checkout detection. remove-module now also runs its dirty-check on worktrees and prunes the main repo's worktree entry after deletion.