bpo-40421: Add Include/cpython/code.h header file (GH-19756)

bpo-35134, bpo-40421: Add Include/cpython/code.h header file.

code.h now defines PyCodeObject type in the limited C API. It is now
included by Python.h.

Give a name to the PyCodeObject structure: it is now called
"struct PyCodeObject". So it becomes possible to define PyCodeObject
as "struct PyCodeObject" in the limited C API without defining the
structure.
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index f54f155..21b51bf 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -131,6 +131,7 @@
     <ClInclude Include="..\Include\cpython\bytearrayobject.h" />
     <ClInclude Include="..\Include\cpython\bytesobject.h" />
     <ClInclude Include="..\Include\cpython\ceval.h" />
+    <ClInclude Include="..\Include\cpython\code.h" />
     <ClInclude Include="..\Include\cpython\dictobject.h" />
     <ClInclude Include="..\Include\cpython\fileobject.h" />
     <ClInclude Include="..\Include\cpython\fileutils.h" />