bpo-40268: Add pycore_runtime.h header file (GH-19493)

Move PyRuntimeState from pycore_pystate.h to pycore_runtime.h.

Remove _PyGILState_check_enabled macro: access directly
_PyRuntime.gilstate.check_enabled.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 95f9e9b..df0eb3a 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -184,6 +184,7 @@
     <ClInclude Include="..\Include\internal\pycore_pylifecycle.h" />
     <ClInclude Include="..\Include\internal\pycore_pymem.h" />
     <ClInclude Include="..\Include\internal\pycore_pystate.h" />
+    <ClInclude Include="..\Include\internal\pycore_runtime.h" />
     <ClInclude Include="..\Include\internal\pycore_sysmodule.h" />
     <ClInclude Include="..\Include\internal\pycore_traceback.h" />
     <ClInclude Include="..\Include\internal\pycore_tupleobject.h" />