Use `yarn` instead of `npm` to install dependencies

[alexjpwalker]

Problem to Solve

We've experienced quite a few issues with npm already - non-deterministic builds, errors that are a nightmare to debug.

Proposed Solution

Based on my experience building Workbase with yarn, I feel like it's more stable and reliable, and more performant. We used to use yarn for client-nodejs 1.8, so the necessary Bazel tooling should already be there in version control, making this change substantially easier.

[alexjpwalker]

This was implemented earlier this year.