Parse Paths in extends attributes

Closes #916

crates/backend/src/ast.rs

@@ -143,7 +143,7 @@ pub struct ImportType {
     pub attrs: Vec<syn::Attribute>,
     pub doc_comment: Option<String>,
     pub instanceof_shim: String,
-    pub extends: Vec<Ident>,
+    pub extends: Vec<syn::Path>,
     pub vendor_prefixes: Vec<Ident>,
 }