Support #[wasm_bindgen(setter, js_name = ...)]

Previously we'd require the explicit `js_name` to *also* start with
`set_`, but when explicitly specified it shouldn't be mangled at all!

Closes #584

crates/macro-support/src/parser.rs

@@ -689,6 +689,7 @@ fn function_from_decl(
         ast::Function {
             name: js_name.map(|s| s.0.to_string()).unwrap_or(decl_name.to_string()),
             name_span: js_name.map(|s| s.1).unwrap_or(decl_name.span()),
+            renamed_via_js_name: js_name.is_some(),
             arguments,
             ret,
             rust_vis: vis,