Implement new shortcut `Shift+Cmd+L` for opening chat

[happinyz]

This PR implements https://github.com/trypear/pearai-app/issues/34.

The previous Cmd+L shortcut essentially did what @Fryingpannn wanted for the new Shift+Cmd+L command. I have not implemented the code selection feature. Let's raise a new issue for that and I or someone else can make a new PR.

The following changes are implemented in this PR:

• If the chat is closed, Cmd+L opens the previously selected chat. If there is no previously selected chat, opens a new chat.
• If the chat is open, Cmd+L does nothing.
• If the chat is closed, Shift+Cmd+L starts and opens a new chat.
• If the chat is open, Shift+Cmd+L starts a new chat.
• New methods to help with selecting/showing the previous chat.
• Documentation for new methods.

https://github.com/trypear/pearai-app/assets/59787978/a5e51e9c-6d5b-4ddb-96df-eec99a83ba88

This is my first time contributing to a VSCode extension, critique and suggestions are welcome!

[happinyz]

Do we have a way to auto fix styling issues? yarn nx lint just warns me I have formatting issues but does nothing to fix them

EDIT: yarn prettier --write './extensions/pearai-extension/**/src/**/*.{ts,tsx}' works!