Implement the local JS snippets RFC

This commit is an implementation of [RFC 6] which enables crates to
inline local JS snippets into the final output artifact of
`wasm-bindgen`. This is accompanied with a few minor breaking changes
which are intended to be relatively minor in practice:

* The `module` attribute disallows paths starting with `./` and `../`.
  It requires paths starting with `/` to actually exist on the filesystem.
* The `--browser` flag no longer emits bundler-compatible code, but
  rather emits an ES module that can be natively loaded into a browser.

Otherwise be sure to check out [the RFC][RFC 6] for more details, and
otherwise this should implement at least the MVP version of the RFC!
Notably at this time JS snippets with `--nodejs` or `--no-modules` are
not supported and will unconditionally generate an error.

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

Closes #1311

crates/backend/src/util.rs

@@ -94,7 +94,7 @@ pub fn ident_ty(ident: Ident) -> syn::Type {
 
 pub fn wrap_import_function(function: ast::ImportFunction) -> ast::Import {
     ast::Import {
-        module: None,
+        module: ast::ImportModule::None,
         js_namespace: None,
         kind: ast::ImportKind::Function(function),
     }