Command line update for TBB

[lofidevops]

User story: I am a user who updates my system form the command line (e.g. with apt update). I want to update my copy of TBB from the command line, so that I can use the same user interface for all my updates.

Proposed workflow:

1. User enters torbrowser-launcher --update
2. Perform startup tasks (e.g. refresh local keyring)
3. Compare installed version to latest TBB version
4. If TBB is up-to-date, exit with an error message "No newer version found"
5. If TBB is not up-to-date, download and install new version

Notes:

• Step 5 could launch the existing GUI and invoke "Reinstall Tor Browser", rather than creating a complete update CLI.