feat: added playground config files (#4669)

* feat: added playground config files

We people might want to try out the main repo as well in stackblitz, we should add it's config file. For codesandbox this is obsolete at the moment, as it has a limit of 500 modules, which we don't fit.

* Update .stackblitzrc

* Update .stackblitzrc

Author: mfranzke

.stackblitzrc

@@ -0,0 +1,3 @@
+{
+  "startCommand": "npm run build && npm run start"
+}