feat: add unimport plugin for Nitro support

[productdevbook]

cc: #531, #732, #677

This supports automatic imports and aliases on the server side.

[danielroe]

Thank you for this.

We probably need this to be an entirely separate vitest environment, as there will be a lot of what the current vitest environment does that shouldn't be done for nitro/server routes.

[productdevbook]

I don't know how to do it the other way around, so don't expect a pr from me. :D But this pr is workable for testing composables like utils etc.

I hope we will soon have the ability to test this nitro.