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/macro/ui-tests/import-local.rs

@@ -0,0 +1,15 @@
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen(module = "./foo.js")]
+extern {
+    fn wut();
+}
+
+#[wasm_bindgen(module = "../foo.js")]
+extern {
+    fn wut();
+}
+
+fn main() {}

crates/macro/ui-tests/import-local.stderr

@@ -0,0 +1,14 @@
+error: relative module paths aren't supported yet
+ --> $DIR/import-local.rs:5:25
+  |
+5 | #[wasm_bindgen(module = "./foo.js")]
+  |                         ^^^^^^^^^^
+
+error: relative module paths aren't supported yet
+  --> $DIR/import-local.rs:10:25
+   |
+10 | #[wasm_bindgen(module = "../foo.js")]
+   |                         ^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+