Add Clang nullability annotations (RBS_NULLABLE/RBS_NONNULL) to C AST

Add RBS_NULLABLE and RBS_NONNULL macros that expand to _Nullable/_Nonnull
on Clang and to nothing on other compilers. Annotate all pointer fields
in generated AST structs and constructor functions based on their
optionality defined in config.yml, and hand-written list/hash data
structures in ast.h/ast.c.

Also enable -Wnullable-to-nonnull-conversion in extconf.rb so that
Clang warns when a nullable pointer is implicitly passed to a nonnull
parameter.

https://claude.ai/code/session_013XvhPzhCAZGQ9qniMY1YQY

Author: claude

ext/rbs_extension/extconf.rb

@@ -16,6 +16,7 @@
   '-Wimplicit-fallthrough',
   '-Wunused-result',
   '-Wc++-compat',
+  '-Wnullable-to-nonnull-conversion',
 ]
 
 if ENV['DEBUG']