chore: use pnpm to manage dependencies (closes #12635)

[thmsobrmlr]

Release notes: Let's make sure to include Jacob here.

Continuing the excellent work of @jacobwgillespie at #12635 here, since apparently GitHub does not allow edits by maintainers if the repo and the fork are located in organization accounts.

Associated Handbook update here: PostHog/posthog.com#4865

Problem

Followup from #12520 (see there for more context), this PR replaces Yarn 1 with pnpm. Yarn 1 is semi-unmaintained and has caused issues in docker build.

Changes

This PR switches to use pnpm instead of yarn. The highlights are:

• All yarn commands are updated to their pnpm equivalents
• GitHub Actions workflows and the Dockerfile use pnpm now
• Some additional dependencies have been added to package.json if they previously were only available due to hoisting.

How did you test this code?

Installed locally + ran docker build. Will need CI to run the tests.

[jacobwgillespie]

Nice, thanks for picking this up, I haven't had many spare cycles to finish the PR lately 🙌

[Twixes]

It works!

[thmsobrmlr]

Nice, thanks for picking this up, I haven't had many spare cycles to finish the PR lately 🙌

Thanks for your awsome work at #12635, @jacobwgillespie! We probably wouldn't have made the switch yet without your contribution and are really happy to have it. I'll make sure to include you in the next release notes :)

[jacobwgillespie]

Awesome, happy to see it merge! 🚀