PERF: Only apply terser to production assets (#24699)

We funnel vendored javascript through ember-cli, but that's only used for the testem environment. Therefore, there's no need to minify it in production builds. In my tests, this reduces peak RSS of a production build from 3.53GB to 3.15GB.

app/assets/javascripts/discourse/ember-cli-build.js

@@ -141,16 +141,20 @@ module.exports = function (defaults) {
     parsePluginClientSettings(discourseRoot, vendorJs, app),
     funnel(`${discourseRoot}/public/javascripts`, { destDir: "javascripts" }),
     generateWorkboxTree(),
+    applyTerser(
       concat(adminTree, {
         inputFiles: ["**/*.js"],
         outputFile: `assets/admin.js`,
-    }),
+      })
+    ),
+    applyTerser(
       concat(wizardTree, {
         inputFiles: ["**/*.js"],
         outputFile: `assets/wizard.js`,
-    }),
+      })
-    generateScriptsTree(app),
+    ),
-    discoursePluginsTree,
+    applyTerser(generateScriptsTree(app)),
+    applyTerser(discoursePluginsTree),
     testStylesheetTree,
   ];
 
@@ -261,5 +265,5 @@ module.exports = function (defaults) {
     },
   });
 
-  return mergeTrees([appTree, applyTerser(mergeTrees(extraPublicTrees))]);
+  return mergeTrees([appTree, mergeTrees(extraPublicTrees)]);
 };