Migrate from yarn to pnpm

[nolanlawson]

We are still using yarn v1 in this repo – we should migrate to a more modern solution.

After internal discussion we decided on pnpm. Two challenges:

1. Volta has experimental pnpm support – developers will have to enable an env variable.
2. The tachometer build script will need to be agnostic to pnpm vs yarn since it might test against older commits that are still using yarn. We can use the presence or absence of yarn.lock to test for this
3. GitHub Actions needs to use whatever version of pnpm we are using in our Volta config

[harshsbhat]

Would like to work on this. Is this issue ready to work on?

[nolanlawson]

@harshsbhat Sure, this is available. Thanks in advance!