Resolve merge conflicts with ChaiScript:develop. Upstream added
nested namespace support (#675), grammar railroad diagrams (#673),
and WASM exception support (#689). Conflicts in chaiscript_common.hpp,
chaiscript_eval.hpp, and chaiscript_parser.hpp resolved by keeping
both Using and Namespace_Block AST node types.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>