Port the OSX-specific platform code to ObjectiveC++

[Duckwhale]

No more wild pointer casting and insane runtime crashes. This is a necessity to actually fix some of the more serious issues, such as the library crashing when multiple windows are created due to failing to reuse a shared app delegate.

[Duckwhale]

The first big hurdle here is that the header-only library must be split because ObjectiveC "contaminates" every compilation unit it touches. Then, here's an entangled web of interdependence, particularly in the Windows platform code (custom WebView2Loader etc.) that. This makes separating the platform-specific code needlessly challenging, as it's referencing the "deprecated" and somewhat questionable JSON RPC functionality and the nondescript detail namespace in general.

All that to say, in the current form the library's code isn't conducive to this kind of sweeping change. Therefore, this is blocked.