add --input flag to powersync-web copy-assets command

[jasonlewicki]

We're on a monorepo (nx), so our node_modules folder could be (is) in a different location. I am using a custom script at the moment to copy those files so this doesnt affect me right now, but others may run into this.

Thanks!

[michaelbarnes]

Thanks @jasonlewicki This is useful information.