[🐧] Linux OS Support

[majockbim]

The r/unixporn community showed some interest in a Linux compatible version of spectrum, which is what motivated this issue.

spectrum is currently Windows-only due to its dependency on WASAPI for system audio capture. ALl other components (FFTW3, ANSI rendering, etc..) are already cross-platform.

Milestones

• Research Linux audio API alternatives (PipeWire, ALSA, PulseAudio) $\leftarrow$ pipewire has been suggested by linux users
• Select and implement audio capture backend
• Abstract audio engine behind a platform interface (AudioEngine) $\leftarrow$ a good method could be refactoring engine.cpp with inheritance
• Verify ANSI/Braille rendering compatibility on Linux terminals
• CMake cross-platform build support
• Test on major distros (Ubuntu, Arch, Fedora)
• Post on r/unixporn