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. (In the future, it may be done more efficiently using runtime.AddCleanup, but doing that would require depending on Go 1.24.0 or higher, which is not desirable yet.)

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