Replace slow Yarn v1 workspace functionality with newer package manager

[JohnAlbin]

According to CONTRIBUTING.md:

We use [Yarn](https://yarnpkg.com) and `yarn workspaces` for development.

When Yarn v1 came out in 2016, it was much faster than npm. But npm, pnpm and Yarn v2 have very similar performance nowadays. See https://yarnpkg.com/benchmarks

However, Yarn v1 is MUCH slower than npm >=5, yarn >=2, etc for cold installs (though it still comparable for other operation types.)

Also, the workspace functionality is more feature-rich in newer package managers compared to yarn v1.

We should investigate which package manager we should migrate to. At one time I did use yarn v1, but I abandoned it when npm performance issues were fixed. I also don't have a strong opinion on the matter.

All the package managers have workspace features: pnpm, npm, yarn