Be sure to exit from resolved promise deleteIfExists(...) helper

Failing to exit when the specified file path doens't exist means that we try to delete that file even though it doesn't exist. This results in a timing issue because the deletion is asynchronous. Essentially, it might unintentionally delete the file springs into being before the promise is resolved.

src/common.ts

@@ -78,7 +78,7 @@ export function deleteIfExists(filePath: string): Promise<void> {
     .then((exists: boolean) => {
         return new Promise<void>((resolve, reject) => {
             if (!exists) {
-                resolve();
+                return resolve();
             }
 
             fs.unlink(filePath, err => {