Implement a polyfill attribute for imports

Allow using imported APIs under alternative names, such as prefixed
names, for web APIs when the exact API differs across browsers.

guide/src/reference/attributes/on-js-imports/polyfill.md

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+# Polyfilling APIs
+
+In JS new APIs often have polyfills via different names in various contexts. For
+example the `AudioContext` API is known as `webkitAudioContext` in Safari at the
+time of this writing. The `polyfill` attribute indicates these alternative
+names.
+
+For example to use `AudioContext` you might do:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(polyfill = webkitAudioContext)]
+    type AudioContext;
+
+    // methods on `AudioContext` ...
+}
+```
+
+Whenever `AudioContext` is used it'll use `AudioContext` if the global namespace
+defines it or alternatively it'll fall back to `webkitAudioContext`.
+
+Note that `polyfill` cannot be used with `module = "..."` or `js_namespace =
+...`, so it's basically limited to web-platform APIs today.