Implement transitive support for NPM dependencies

This commit implements [RFC 8], which enables transitive and transparent
dependencies on NPM. The `module` attribute, when seen and not part of a
local JS snippet, triggers detection of a `package.json` next to
`Cargo.toml`. If found it will cause the `wasm-bindgen` CLI tool to load
and parse the `package.json` within each crate and then create a merged
`package.json` at the end.

[RFC 8]: https://github.com/rustwasm/rfcs/pull/8

crates/shared/src/lib.rs

@@ -17,6 +17,7 @@ macro_rules! shared_api {
             local_modules: Vec<LocalModule<'a>>,
             inline_js: Vec<&'a str>,
             unique_crate_identifier: &'a str,
+            package_json: Option<&'a str>,
         }
 
         struct Import<'a> {