Jest compatibility with Sveltekit runtime modules

[rossyman]

There's a wider ongoing discussion around compatibility with Sveltekit runtime modules outside of a build context, i.e.: when testing. This issue leads on from #14, which was initially intended to be specifically about Vite imports, and allows us to track the progress of https://github.com/sveltejs/kit/issues/1485 and ultimately determine whether we wish to add direct support to the adder to enable developers to mock runtime modules, or just provide documentation on how to do so theirselves.

You can see currently proposed solutions here:

• https://github.com/rossyman/svelte-add-jest/issues/14#issuecomment-916377903