Fix isReservedIdentifier (#361)

Switches `isReservedIdentifier` to use `hasOwnProperty` to avoid
matching the Object prototype.

src/LuauAST/util/isReservedIdentifier.ts

@@ -1,5 +1,5 @@
 import { globals } from "LuauAST/impl/globals";
 
 export function isReservedIdentifier(id: string) {
-	return id in globals;
+	return Object.prototype.hasOwnProperty.call(globals, id);
 }