`--header` switch is deprecated. Use nimcache?

[geekrelief]

Instead of generating and copying over NimForUEFFI.h, try using nimcache to build.

touches:
config.nims has switch("header", "NimForUEFFI.h") when compiling the host
Source/NimForUE/NimForUE.Build.cs uses the "NimHeaders" folder in AddNimForUEDev
Source/NimForUE/Private/NimForUE.cpp includes NimForUEFFI.h
Source/NimForUE/Private/TestActor.cpp includes NimForUEFFI.h

[geekrelief]

For now --header is the easiest way for Unreal to get access to functions from nim without having to hand write the exported functions from a dll. When removing the host dll, we can integrate the nimcache cpp files into the NimForUE to compile with the unreal code.