build: Swapping NPM for PNPM Step 4 of 5

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Motivations

1. We've long wanted to explore pnpm, and between some of the new features they're rolling out, the fact that Storybook is embracing it fully, and some of our internal packages have as well.
2. This PR Chain is in 5 steps, to make it easier to review.
3. This fourth PR is the actual PNPM migration.
4. It needs to be tested the most, as it's the actual change.
5. In this branch, everything except for CircleCI should be functional (that's the next PR).
6. You should be able to run the docs site, you will have to delete your local node_modules folders when moving into this branch. You will need to have pnpm installed already.

Changes can be
tested via Pre-release

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