fix(@angular-devkit/build-angular): disable top level variable and function name mangling

Disables toplevel mangling and enables compression for es5 bundles. With compress enabled we reduce a further ~7Kb

Closes: #15436

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/process-bundle.ts

@@ -132,11 +132,10 @@ async function processWorker(options: ProcessBundleOptions): Promise<void> {
 
     // Mangle downlevel code
     const result = minify(code, {
-      compress: false,
+      compress: true,
       ecma: 5,
       mangle: true,
       safari10: true,
-      toplevel: true,
       output: {
         ascii_only: true,
         webkit: true,