npm run scripts: wait for the spawned process to finish

Using `wait` is beneficial as (1) it shows the clear intention of running programs in parallel to the reader and (2) the CLI actually waits for the processes to finish and not suddenly dumps texts after a few seconds.

See also: http://tldp.org/LDP/abs/html/x9644.html

Author: wilzbach

package.json

@@ -8,8 +8,8 @@
     "build:js-dev": "webpack --mode development",
     "build:js": "webpack --mode production",
     "build:py": "node ./extract-meta src/lib/components > my_dash_component/metadata.json && copyfiles package.json my_dash_component && python -c \"import dash; dash.development.component_loader.generate_classes('my_dash_component', 'my_dash_component/metadata.json')\"",
-    "build:all": "npm run build:js & npm run build:py",
-    "build:all-dev": "npm run build:js-dev & npm run build:py"
+    "build:all": "npm run build:js & npm run build:py; wait",
+    "build:all-dev": "npm run build:js-dev & npm run build:py; wait"
   },
   "author": "my-name my-email",
   "license": "my-license",