Convert build scripts to ESM (#38984)

build/postcss.config.mjs

@@ -0,0 +1,17 @@
+const mapConfig = {
+  inline: false,
+  annotation: true,
+  sourcesContent: true
+}
+
+export default context => {
+  return {
+    map: context.file.dirname.includes('examples') ? false : mapConfig,
+    plugins: {
+      autoprefixer: {
+        cascade: false
+      },
+      rtlcss: context.env === 'RTL'
+    }
+  }
+}