Add config file option to set default scope flag

[mattrunyon]

💡 Feature description

From what I can find in the docs, the only way to set the package scope when building is via the CLI flag. This could be error prone in a repo as any script that calls build for different reasons needs to specify the scope and not mess it up.

This is contrasted by the package name being read from Cargo.toml. Scope is effectively part of the package name in npm. I don't think I've ever seen publishing the same package under multiple scopes unless the organization changed their name/scope and the old one is marked as deprecated. To me, it doesn't make sense that scope would not be part of Cargo.toml just like name is.

💻 Basic example

Something like this should produce a package.json with @my-scope/my-package as the package name when running wasm-pack build without the --scope flag. I have little experience in Rust config, so no idea if package is the right section here

[package]
name = "my-package"
scope = "my-scope"