[static-build] add support for gatsby esm config (#9183)

Adds support for auto injection into Gatsby's new ESM config format. Only injects if the `gatsby-config.mjs` is present, and continues to default generation of `gatsby-config.js`

packages/static-build/test/builds.test.js

@@ -290,6 +290,54 @@ it(
   FOUR_MINUTES
 );
 
+it(
+  'Should build Gatsby with configuration defined in esm',
+  async () => {
+    const { workPath } = await runBuildLambda(
+      path.join(__dirname, 'build-fixtures/14-gatsby-with-esm-config')
+    );
+
+    const contents = await fs.readdir(workPath);
+
+    expect(contents.some(name => name === 'gatsby-config.js')).toBeFalsy();
+    expect(contents.some(name => name === 'gatsby-config.ts')).toBeFalsy();
+    expect(contents.some(name => name === 'gatsby-config.mjs')).toBeTruthy();
+    // using `import` causes a seg fault.
+    expect(await fs.readFile(path.join(workPath, 'gatsby-config.mjs'), 'utf-8'))
+      .toBe(`import userConfig from "./gatsby-config.mjs.__vercel_builder_backup__.mjs";
+
+// https://github.com/gatsbyjs/gatsby/blob/354003fb2908e02ff12109ca3a02978a5a6e608c/packages/gatsby/src/bootstrap/prefer-default.ts
+const preferDefault = (m) => (m && m.default) || m;
+
+const vercelConfig = Object.assign(
+  {},
+
+  // https://github.com/gatsbyjs/gatsby/blob/a6ecfb2b01d761e8a3612b8ea132c698659923d9/packages/gatsby/src/services/initialize.ts#L113-L117
+  preferDefault(userConfig)
+);
+if (!vercelConfig.plugins) {
+  vercelConfig.plugins = [];
+}
+
+const hasPlugin = vercelConfig.plugins.find(
+  (p) =>
+    p && (p === "gatsby-plugin-vercel" || p.resolve === "gatsby-plugin-vercel")
+);
+
+if (!hasPlugin) {
+  vercelConfig.plugins = vercelConfig.plugins.slice();
+  vercelConfig.plugins.push({
+    resolve: "gatsby-plugin-vercel",
+    options: {},
+  });
+}
+
+export default vercelConfig;
+`);
+  },
+  FOUR_MINUTES
+);
+
 it(
   'Should build Nuxt.js with "@nuxtjs/web-vitals" plugin',
   async () => {