Rewrite Vitest adder built-in to svelte-add

[rgossiaux]

Would be nice if there was a vitest integration. I tried setting vitest up myself and couldn't get the config working properly, so I'm sure it'd save folks a lot of time if they wanted to try using vitest.

[wd-David]

I've recently tackled some problems when testing different svelte packages in Vitest: janosh/svelte-multiselect#48.
My DX improves a ton after I switched from Jest to Vitest, so I'm considering writing an adder for that.
Here is an example you can try online.

[david-plugge]

didnt see this issue, oops
#212

[babichjacob]

@davipon has created an external adder! Check it out. Renaming this issue accordingly.

[wd-David]

@benmccann had created a discussion here: sveltejs/kit#5285
To make Vitest an option in create-svelte.

[manuel3108]

create-svelte is not handled by svelte-add, because it's an official package from the core svelte team. Adiitionally this is currently supported by the main create-svelte cli