test(sveltekit): Switch to vitest

[AbhiPrasad]

Sveltekit uses is esm only and uses conditional exports, which doesn't play nicely with our current version of jest.

 FAIL  test/server/load.test.ts
  ● Test suite failed to run

    Cannot find module '@sveltejs/kit' from 'test/server/load.test.ts'

       7 | // @ts-ignore sdf
       8 | // eslint-disable-next-line import/no-unresolved
    >  9 | import * as Kit from '@sveltejs/kit';
         | ^
      10 |
      11 | // Kit.error = jest.fn();
      12 | import { wrapLoadWithSentry } from '../../src/server/load';

      at Resolver._throwModNotFoundError (../../node_modules/jest-resolve/build/resolver.js:427:11)
      at Object.<anonymous> (test/server/load.test.ts:9:1)

We can't update jest for now, so swapping to use vitest instead.

Bonus is that vitest is waaaaay faster, but it is restricted to Node 14+ (which we are fine for Sveltekit).

[Lms24]

Nice!