FEAT: introduce sidePanel that can always stay active and updates when tab is created/changed/deleted

[crebro]

CHANGES:

1. create sidePanel and make it the default when the user clicks on the extension in the extensions menu
2. register command to open sidePanel ( Cmd + Shift + U ) or ( Ctrl + Shift + U )
3. Send state messages on create/update/delete tabs from background.js to update the sidePanel live
4. update sidePanel in specific ways in response to the state messages from background.js so that the entire loading-tabs-from-scratch process doesn't have to run every time.

TODO & FIXES required:

• Write comprehensive comments
• [BUG] When the user creates a new tab while the current tab is opening/loading, a duplicate item is seen on the sidePanel.