add isESM helper

Author: lobsterkatie

packages/nextjs/src/utils/isESM.ts

@@ -0,0 +1,17 @@
+/**
+ * Determine if the given source code represents a file written using ES6 modules.
+ *
+ * The regexes used are from https://github.com/component/is-module, which got them from
+ * https://github.com/formatjs/js-module-formats, which says it got them from
+ * https://github.com/ModuleLoader/es-module-loader, though the originals are now nowhere to be found.
+ *
+ * @param moduleSource The source code of the module
+ * @returns True if the module contains ESM-patterned code, false otherwise.
+ */
+export function isESM(moduleSource: string): boolean {
+  const importExportRegex =
+    /(?:^\s*|[}{();,\n]\s*)(import\s+['"]|(import|module)\s+[^"'()\n;]+\s+from\s+['"]|export\s+(\*|\{|default|function|var|const|let|[_$a-zA-Z\xA0-\uFFFF][_$a-zA-Z0-9\xA0-\uFFFF]*))/;
+  const exportStarRegex = /(?:^\s*|[}{();,\n]\s*)(export\s*\*\s*from\s*(?:'([^']+)'|"([^"]+)"))/;
+
+  return importExportRegex.test(moduleSource) || exportStarRegex.test(moduleSource);
+}