Document how to use TypeScript

[fregante]

The page https://svelte.dev/docs/typescript#setup-other-build-tools points users to the root of this repo, which lacks any mention of TypeScript entirely.

It would be great to see how to configure the preprocessor to enable TS support

[fregante]

In my case all I needed was:

import { sveltePreprocess } from 'svelte-preprocess';

and

svelte({
	preprocess: sveltePreprocess()
})