[gatsby-plugin-vercel-builder] Use "esbuild" to build package (#10508)

Removes the two-staged `tsc` build into a single `esbuild` bundle. The `ssr-handler.ts` template file is moved to the root of the package and converted to JavaScript.

packages/gatsby-plugin-vercel-builder/gatsby-node.js

@@ -0,0 +1,8 @@
+const { generateVercelBuildOutputAPI3Output } = require('./dist/index.js');
+
+exports.onPostBuild = async ({ store }) => {
+  await generateVercelBuildOutputAPI3Output({
+    // validated by `pluginOptionSchema`
+    gatsbyStoreState: store.getState(),
+  });
+};