Use `npx degit` Instead Of `polywrap create`

I propose we use `degit` instead of `polywrap create`, as they effectively function the same way. This means we maintain less code, and can use a popular widely-used package: https://www.npmjs.com/package/degit

In this future it could be `pwr degit` instead of `npx degit`. Also for a nice UX, we could keep the name `polywrap create` but just use `degit` underneath.