Migration away from Yarn -> pnpm

[tobyab]

Build + install time with Yarn takes a long time. The structure of the lockfile can commonly result in conflicts, too. We should consider migrating to pnpm. Build + install times are much faster (pnpm.io/benchmarks), and conflicts are much easier to mitigate & resolve with the structure of the lockfile.

[maxwofford]

#1082 (comment)