fix: use defineProperty for appending prop in storeName (#194)

src/MagicString.js

@@ -365,7 +365,7 @@ export default class MagicString {
 
 		if (storeName) {
 			const original = this.original.slice(start, end);
-			this.storedNames[original] = true;
+			Object.defineProperty(this.storedNames, original, { writable: true, value: true, enumerable: true });
 		}
 
 		const first = this.byStart[start];