Use pnpm package manager instead of yarn

[BlueCutOfficial]

This PR would use pnpm instead of yarn as a package manager.

Relates to #1117:

Conversion to V2 addon is a quite big change that can be hard to debug and review. Since #1117 is already quite old, it would probably be easier to get back to this by splitting the work: first move to pnpm, then work on the monorepo structure to separate the V1 addon package and the test app, then properly convert to V2. I couldn't reuse the commit moving to pnpm since it wasn't isolated from other changes, but maybe #1117 could simply be rebased in the future.

Notes:

• The failing tests are the ones that are already failing on master, so I assumed I could open for review as it is.
• volta-cli/action@v4 doesn't seem to support pnpm at the moment, unless it's under-documented. It seems that work was done on volta-cli to support pnpm, but the related issue is not closed.

[elwayman02]

Thanks! This is a great start.

[elwayman02]

I'm not too worried about the failing tests right now, as you mentioned they're already failing on the main branch. We can honestly drop the Ember 4.4 tests - I want to keep Ember 4.12 as a scenario as well as the latest Ember 5 release, but we don't need the other old scenarios. That's a change we can probably make a long the way to the V2 addon.