* Remove ObjCRuntime.nfloat (in favor of System.Runtime.InteropServices.NFloat).
* Automatically add a reference to the System.Runtime.InteropServices.Internal
package, so that developers get the new NFloat API (with operators) we've
added post .NET 6 (but don't do this for .NET 7).
* Automatically add a global using alias for
System.Runtime.InteropServices.NFloat -> nfloat. This is not behind the
usual `ImplicitUsings` condition our other implicit usings are, because
they're off by default for existing projects, and the main target for the
global using alias for nfloat is upgraded projects.
* Automatically generate a global using alias (like above) in the generator
for all code the generator compiles.
* Update xtro entries to reference System.Runtime.InteropServices.NFloat
instead of ObjCRuntime.nfloat.
* Add a workaround for a hopefully temporary issue with .NET/CoreCLR where the
wrong runtime pack is selected otherwise (without the new NFloat API, so
nothing works at runtime).
Ref: https://github.com/xamarin/xamarin-macios/issues/13087
bd97933b60