Typechecking, intellisense etc

[Rich-Harris]

Not gonna lie, I have mostly no idea what is involved in this. But it would be cool if we could get red squigglies for code like this...

/** @type {number} */
const num = 'definitely not a number';
const doubled = num * 2;

...and if stuff in node_modules could get resolved by Monaco (including svelte, svelte/store etc, which currently resolve correctly, but only because the app itself has a dependency on them).

Also, can we get the editor to behave more like VSCode with the Svelte extension? At the moment it's just treating Svelte components as HTML files, which sorta works but isn't a viable long term solution.

[dummdidumm]

Related sveltejs/language-tools#1190