Fix #28: add regex support

[leftibot]

Automated fix by @leftibot.

What changed

Fix #28: Add std::regex support as a ChaiScript extras module
Adds a new header-only regex module exposing std::regex, std::smatch,
regex_search, regex_match, and regex_replace to ChaiScript. Includes
support for regex flags (e.g. icase) and match result access via
smatch indexing. Follows the existing module pattern used by math
and string_methods.
Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Files

 include/chaiscript/extras/regex.hpp | 70 +++++++++++++++++++++++++++++++++++++
 tests/CMakeLists.txt                |  6 ++++
 tests/regex.cpp                     | 61 ++++++++++++++++++++++++++++++++
 3 files changed, 137 insertions(+)

Closes #28

Triggered by @lefticus.