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.

crates/shared/src/lib.rs

@@ -87,6 +87,7 @@ pub struct ImportStatic {
 pub struct ImportType {
     pub name: String,
     pub instanceof_shim: String,
+    pub polyfills: Vec<String>,
 }
 
 #[derive(Deserialize, Serialize)]