Defer posthog script loading to relieve js main thread and improve site vitals

Unclear what exactly causes the bad web vitals, this should be investigated in the future. I tried to set the scripts async, but it didn't help. Interestingly enough, "array.js", which is initiated by the cdn script, is already set as async, meaning it shouldn't block anything at all!

I'm unsure if we can even fix this and if this is an issue on the posthog side or a misinterpretation on the Google side.
Thanks for the report and let's keep an eye on this, but I think we should treat it as minor for now.

Our implementation fits the best practices from posthog, I think.