chore: Migrate to pnpm and modularise Nx tasks

[lachlancollins]

Changes:

• pnpm import to clone existing locked versions
• Setup pnpm workspaces
• Remove yarn.lock and lerna
• Split tsc and vitest into per-package scripts rather than root-level scripts (can be parallelised by Nx)
• Fix broken react dependency resolution
• Replace rm -rf (unix-specific) with rimraf

CI testing time:

• Before: 42s
• After: 31s

[lachlancollins]

@KevinVandy I've made some great progress here! I reckon I'll be able to submit the tsup PR after this.

[KevinVandy]

I've tried to find anything broken with this, but don't see anything. Thanks for the hard work. We'll see if publishing still works