Add user-config.jam to set MSVC compiler heap size globally for b2

Sets /Zm500 flag for MSVC toolset at the b2 configuration level, ensuring
it applies to all builds including CI. This complements the Jamfile-level
flag and ensures CI builds get the increased heap allocation.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: jll63

user-config.jam

@@ -0,0 +1,4 @@
+# Boost.Build user configuration for MSVC compiler heap settings
+# This file is read by b2 before processing Jamfiles and applies globally
+
+using msvc : : : <cxxflags>/Zm500 ;