Migrate from `yarn` to `pnpm`

[guidanoli]

📚 Context

We've been using yarn to manage JavaScript/TypeScript dependencies since the beginning. There are other Node package managers available. One of those is pnpm. There are many reasons to migrate from yarn to pnpm. See motivations.

✔️ Solution

Migrate to pnpm

📈 Subtasks

• [ ] Delete the yarn.lock file
• [ ] Run pnpm i to create a pnpm-lock.yaml file
• [ ] Fix GitHub workflow files to use pnpm
• [ ] Fix commands in the README