Rename .mjs files to .js (#14605)

scripts/build/transform/index.js

@@ -2,7 +2,7 @@ import { parse } from "@babel/parser";
 import { traverseFast as traverse } from "@babel/types";
 import babelGenerator from "@babel/generator";
 import { outdent } from "outdent";
-import { SOURCE_DIR } from "../../utils/index.mjs";
+import { SOURCE_DIR } from "../../utils/index.js";
 import allTransforms from "./transforms/index.js";
 
 const generate = babelGenerator.default;