Migrate to a monorepo structure

[charlesBochet]

Closes #2786

We will likely need to iterate quickly to fix all consequences (docs, build, dev environment setup)

What:

• switch to yarn 4 but keeps node_modules linker ; I was unable to find a smooth Dev xp with pnp and vscode
• setup nx at root folder
• setup yarn workspaces and move everything under packages

I've tested the following commands successfully:

• yarn nx run twenty-docs:start
• yarn nx run twenty-front:dev
• yarn nx run twenty-server:start:dev