Use runtime.Pin to remove unsafe code

[jchv]

Time has passed and now it should be trivially possible to fix our broken reliance on pointers not moving, as runtime.Pin eventually did arrive in Go 1.21.

https://github.com/jchv/go-webview2/blob/master/pkg/edge/chromium.go#L49-L59

We probably just need one pinner for all of the handlers. One open question is when to execute the unpin; a finalizer might be the safest bet.

I'm considering this a bug since the code is relying on behavior that is explicitly not guaranteed.