test: migrate preact/utils to vitest

[43081j]

This also adds preact/utils to the pnpm workspace so it can be
resolved properly via @preact/signals-utils.

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[43081j]

we didn't actually need this in the end i think, because we're misusing sub-packages

we define preact/utils as its own package, @preact/signals-utils
but within this repo, we import it as if its a sub-path, @preact/signals/utils.

we don't actually publish this package afaict so this worked fine. but i wonder if we should update it to treat it as an actual package, i.e. change the imports and tsconfig refs to @preact/signals-utils

[JoviDeCroock]

I'd prefer not to, we use this pattern in the main preact repo as well

[43081j]

i will remove this change

it seems like signals/utils having a package.json doesn't make much sense, though. it should probably just be a subdir of signals. we don't expose it to pnpm, so its dependencies don't get install afaik, and nothing resolves to it as everything accesses it by subdir instead.

one for another time though

[43081j]

ok it turns out we do need the pnpm-workspace change @JoviDeCroock

in order for the tests in the utils package to resolve bare specifiers, the utils package itself needs to be picked up by pnpm. so it has to be in the workspace file. but i haven't touched the imports, etc.

i've also downgraded vite in this PR which should fix the failing CI (vite 7 doesn't work in node 18)

we should merge this one before the other PRs