V3

[YousefED]

This PR ships "V3" of TypeCell, which includes a number of significant updates:

• Block editor: change from "Notebook style cells" to incorporating BlockNote for a block-based rich editing experience
• Workspaces: Users will have a workspace and can add documents easily via the sidebar.
• Backend: switch from Matrix-style backend to postgres + HocusPocus
• Local-first: All data will be stored locally and accessed "local first". This also opens up the path to an installable Progressive Web App (PWA) that can work offline.