Use Bun instead of Pnpm

[PaulRBerg]

Pnpm has compatibility issues because of the way it caches the dependencies in node_modules/.pnpm:

• etherscan verification failure (when remapping points to external file) foundry-rs/foundry#5307 (comment)
• Installing dependencies with pnpm instead of git submodules #508 (reply in thread)

We should look into alternatives, and Bun looks promising:

https://bun.sh/package-manager

[PaulRBerg]

Can confirm that Bun is the way to go.

• https://twitter.com/PaulRBerg/status/1741435292521762889
• build: switch to bun for dep management #775