(internal): pnpm migration

[dsinghvi]

This PR migrates fern to use pnpm instead of yarn. Couple changes:

• Commands must be prefixed with pnpm (e.g. pnpm install).
• To filter to a workspace you can run pnpm --filter <package-name> comamnd
• About 50% of workspaces are now using vitest
• mrlint is now deleted and completely removed from the repo (we will have to manage tsconfigs on our own)
• pnpm test runs all unit tests and is more performant (you should be able to run these locally).