The vite package used the node test runner. This PR switches it over to vitest.

The transition was not difficult.

1. before and after for the node test runner have their vitest equivalents in beforeAll and afterAll. See: https://nodejs.org/api/test.html#afterfn-options where after is described as acting like afterAll.
2. Usage of assert was not too difficult to transition as they could be switched to .toEqual and .not.toEqual
3. Mocking of fs was achieved using spys and mockImplementationOnce. There was no clear need to use Once but it felt safer to assume that rather than continue to return the mock value for eternity.

@dac09 As requested I've switched this one out. One of the ts expect error comments is gone but one remains. From what I can see it seems to be a valid complaint - the type of PluginOption looks pretty funky.