add SLANG_USER_CONFIG (#6584)

* add SLANG_USER_CONFIG

* format code

---------

Co-authored-by: Simon Kallweit <skallweit@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Simon Kallweit <64953474+skallweitNV@users.noreply.github.com>

Author: 3 people

include/slang.h

@@ -1,6 +1,10 @@
 #ifndef SLANG_H
 #define SLANG_H
 
+#ifdef SLANG_USER_CONFIG
+    #include SLANG_USER_CONFIG
+#endif
+
 /** \file slang.h
 
 The Slang API provides services to compile, reflect, and specialize code