Replace `yarn` package manager with `pnpm`

[spwoodcock]

• For consistency of package managers across TM tools.
• FMTM uses pnpm, TM will likely be migrated to pnpm after Migrate Frontend codebase to Typescript and Vite tasking-manager#6715
• It's not so clear cut anymore which is best out of pnpm and yarn (speed-wise at least), but pnpm is pretty damn good.