Config: Moves non-OS specific configuration loading to FEXCore

Puts the visibility of the main layer, application layers, and
environment in to FEXCore instead of FEX.
These layers aren't specific to FEX/FEXLoader and should live in
FEXCore.

Only the EmptyMapper remains in FEX, which should eventually move over
to FEXConfig since that is the only user.

Source/Tools/FEXConfig/Main.cpp

@@ -90,7 +90,7 @@ namespace {
     }
     ConfigOpen = true;
     ConfigFilename = Filename;
-    LoadedConfig = std::make_unique<FEX::Config::MainLoader>(Filename);
+    LoadedConfig = FEXCore::Config::CreateMainLayer(&Filename);
     LoadedConfig->Load();
     return true;
   }