feat: add basic Emscripten support

[LimiNode]

Summary

• detect Emscripten in CMake and disable unsupported backends
• run tasks with emscripten_async_call when pthreads are unavailable
• forward console logs to JS console in Emscripten builds
• optionally parse ANSI colors in the browser via a single EM_JS helper
• add Node smoke test for Emscripten builds
• add async flush test to ensure wait() drains the queue
• run Emscripten smoke tests in CI via setup-emsdk

Testing

• cmake -S . -B build
• cmake --build build --target file_logger_test
• cmake --build build
• ctest --test-dir build
• emcmake cmake -S . -B build-ems -DLOG_IT_CPP_BUILD_TESTS=ON -DLOGIT_EMSCRIPTEN=ON -DLOGIT_FORCE_ASYNC_OFF=ON
• cmake --build build-ems --target ems_console ems_async_flush
• node --no-experimental-fetch build-ems/tests/ems_console.js
• node --no-experimental-fetch build-ems/tests/ems_async_flush.js

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https://chatgpt.com/codex/tasks/task_e_68c6ce019ce0832ca52cad725740541e