remove shelljs?

[benmccann]

Do we need an extra dependency for shelljs? It has almost 20 transitive dependencies, a couple of which are deprecated.

Here's the code:
https://github.com/sveltejs/svelte.dev/blob/main/apps/svelte.dev/scripts/update.js

It was added in:
sveltejs/svelte#5262

It seems like we could just do Promise.all without having the shell package in there.

I'm not exactly sure what update_template.js is. I think it's for the now archived sveltejs/template, but it also looks like it generates a file used in AppControls.svelte and I have no idea what it's doing:
https://github.com/sveltejs/svelte/blob/c42bb04276af0024b49aa46918eec69ad56570a5/sites/svelte.dev/src/routes/(authed)/repl/%5Bid%5D/AppControls.svelte#L147