feat(dev): dont override asset manifest in dev-server (#11648)

Override manifest.json only when `npm run dev`, not `npm run
dev-server`.  This allows developers to compare between the
current version of Superset with what they are working on
without switching back and forth between branches.

All they need to do is to first start the current version with
production build:

```
npm run build
superset run -p 8088
```

Then start the dev server in another window:

```
npm run dev-server
```

superset-frontend/webpack.config.js

@@ -47,6 +47,7 @@ const {
   nameChunks = false,
 } = parsedArgs;
 const isDevMode = mode !== 'production';
+const isDevServer = process.argv[1].includes('webpack-dev-server');
 
 const output = {
   path: BUILD_DIR,
@@ -94,17 +95,9 @@ const plugins = [
         entrypoints: entryFiles,
       };
     },
-    // Also write to disk when using devServer
-    // instead of only keeping manifest.json in memory
-    // This is required to make devServer work with flask.
-    writeToFileEmit: isDevMode,
-  }),
-
-  // create fresh dist/ upon build
-  new CleanWebpackPlugin({
-    dry: false,
-    // required because the build directory is outside the frontend directory:
-    dangerouslyAllowCleanPatternsOutsideProject: true,
+    // Also write maniafest.json to disk when running `npm run dev`.
+    // This is required for Flask to work.
+    writeToFileEmit: isDevMode && !isDevServer,
   }),
 
   // expose mode variable to other modules
@@ -126,9 +119,21 @@ const plugins = [
     ],
   }),
 ];
+
 if (!process.env.CI) {
   plugins.push(new webpack.ProgressPlugin());
 }
+
+// clean up built assets if not from dev-server
+if (!isDevServer) {
+  plugins.push(
+    new CleanWebpackPlugin({
+      // required because the build directory is outside the frontend directory:
+      dangerouslyAllowCleanPatternsOutsideProject: true,
+    }),
+  );
+}
+
 if (!isDevMode) {
   // text loading (webpack 4+)
   plugins.push(