refactor(frontend): switch to pnpm

[ravenclaw900]

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[ravenclaw900]

Trying to switch to Yarn v3 (apparently it's v3 now) with Plug n' Play has admittedly given me a lot of trouble. Vite doesn't seem to be compatible with it: vitejs/vite#8357, and our strange workspace layout (with all frontend code located in the frontend directory doesn't seem to be really compatible either. The way I see it right now, we have 3 other options:

1. Use Yarn v3 without PnP: This still has many of the problems with switching away from Yarn classic in the first place, for instance bad Dependabot support and breaking support with most tools that use (what used to be) package manager-agnostic solutions, and isn't that much of an improvement over Yarn classic.
2. npm: Better tooling support and official, but still slower than Yarn v3 without PnP, and I've had problems with it in the past.
3. pnpm: The solution that I'm leaning towards right now. Generally better support from everything, seems to solve some of the dependency issues that I've had in the past, and speed is in the middle of Yarn v3 (PnP) and npm. This would also allow us to go back to Dependabot, which I honestly prefer due to its better GitHub integration (Renovate still hasn't merged chore(deps): update dependency @sveltejs/vite-plugin-svelte to v1.0.1 #305).
   EDIT: Apparently pnpm also supports Plug n' Play (https://pnpm.io/feature-comparison), though it isn't required.