chore: add a Vitest workspace config

[teemingc]

This PR adds a Vitest workspace config for the monorepo so that the Vitest IDE extension works correctly instead of complaining that it's found too many projects.

EDIT: there's an issue caused by SvelteKit resolving the wrong current working directory when running the Vitest extension

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[benmccann]

Removing the use of the cwd was done in the version-3 branch here: #15434

That PR contained a lot of the changes here and there's a lot of conflicts on this PR, so I wonder if we should close this one? It might be easier just to start over on version-3 branch where I imagine the change should be a lot smaller if you want to keep going with this