Build standalone on prepublish

packages/babel-standalone/scripts/prepublish.js

@@ -0,0 +1,7 @@
+// This file is executed by lerna before publishing @babel/standalone,
+// so that it has the new version and not the old one.
+
+require("child_process").execSync("make prepublish-build-standalone", {
+  cwd: require("path").resolve(__dirname, "../../.."),
+  stdio: "inherit",
+});