Move to pnpm

[BobrImperator]

• removes yarn and uses pnpm instead
• adds @embroider/test-setup to all packages because of symlinked ember-try scenarios that attempt to use it.

Note:
ember-release scenario should start passing after #2565

[marcoow]

I’d suggest keeping Volta - that has beta pnpm support now

[BobrImperator]

@marcoow It'd seem that this volta issue is blocking volta-cli/action#121
Also CrowdStrike/ember-headless-table#184

It doesn't seem to me that the experimental support is worth the hassle of figuring it out.

[marcoow]

We're using it in Gravity and (I think) it works quite well – https://github.com/mainmatter/gravity/blob/master/.github/workflows/ci.yml

[BobrImperator]

ember-lts-4.12 seems to be the only version failing and might be bugged, see emberjs/data#8609

[BobrImperator]

Merged since it seems like I've cleared out the concerns, I'd like to see if renovate works correctly and so we can merge new package versions without the need to manually maintain lock files in this PR anymore.