bpo-40241: Add pycore_interp.h header (GH-19499)

Move PyInterpreterState and related functions to a new internal
pycore_interp.h header file.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index df0eb3a..c35499e 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -177,6 +177,7 @@
     <ClInclude Include="..\Include\internal\pycore_hamt.h" />
     <ClInclude Include="..\Include\internal\pycore_import.h" />
     <ClInclude Include="..\Include\internal\pycore_initconfig.h" />
+    <ClInclude Include="..\Include\internal\pycore_interp.h" />
     <ClInclude Include="..\Include\internal\pycore_object.h" />
     <ClInclude Include="..\Include\internal\pycore_pathconfig.h" />
     <ClInclude Include="..\Include\internal\pycore_pyerrors.h" />