Add github actions

[lefticus]

We need automated testing of the ChaiScript_Extras repository

[lefticus]

@leftibot add automated testing following the pattern set in ChaiScript/ChaiScript.

• clean, simple
• MSVC/Windows, AppleClang/MacOS, GCC/Linux
• Add a reasonable combination of ASAN/UBSan targets with Release/Debug.

[lefticus]

@leftibot the build is failing due to outdated cmake code. Investigate, fix, and open a new PR

[leftibot]

Investigated the red CI on master from #34. The "outdated CMake code" has two distinct symptoms, both rooted in the ChaiScript 6.1.0 subproject that gets pulled in via FetchContent:

1. macOS Configure fails — runners now ship CMake 4.x, which removes support for cmake_minimum_required(VERSION < 3.5). The fetched ChaiScript 6.1.0 CMakeLists.txt:1 still says VERSION 2.8, so configure aborts:

CMake Error at build/_deps/chaiscript-src/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

2. Windows MSVC Build fails — ChaiScript 6.1.0's src/main.cpp (the chai REPL) has a missing <chrono> include and no longer compiles on current MSVC ('high_resolution_clock': is not a class or namespace name). That target is declared unconditionally in the subproject, so we were building it even though chaiscript_extras only needs ChaiScript's headers.

Fix (#35): in cmake/chaiscript.cmake, before add_subdirectory, set CMAKE_POLICY_VERSION_MINIMUM 3.5 to unblock the stale cmake_minimum_required, and pass EXCLUDE_FROM_ALL so the upstream chai REPL (and samples/modules) stay out of the default build. Also bumped our own cmake_minimum_required to 3.16 and opted into CMP0169 OLD to silence the FetchContent_Populate deprecation on CMake 3.30+.

Verified locally (CMake 3.31) — Debug and Release both configure, build, and pass tests; the chai binary is no longer built.

Not fixed here — the Linux ASAN/UBSAN job still fails on string_methods_test with a heap-use-after-free in chaiscript::extras::string_methods::split when accessed via a chained index expression ("...".split(",")[1]). That's a runtime bug in the extras+ChaiScript 6.1.0 interaction, not a CMake one. Worth tracking as a separate issue — do you want me to open it?

PR: #35