Add support for specifying the target generator config file

src/fixtures/gatsby/default.expected.cjs

@@ -0,0 +1,8 @@
+module.exports = {
+  pathPrefix: "/docs/",
+  siteMetadata: {
+    title: `My Gatsby Site`,
+    siteUrl: `https://www.yourdomain.tld`,
+  },
+  plugins: [],
+}