[examples] Added: audio_fft_spectrum_visualizer example

[meisei4]

Here is if the FFT part of the audio_wave/spectrum_visualizer - wave and frequency spectrum visualization (FFT)

We could add a second example of how to do waveform visualization, but i think that FFT and the resources provided in this example are enough to inspire for users making their own waveform visualizer with a second row in the texture.

shaders i worked with for this test (to test webaudio constants tweaks against): https://www.shadertoy.com/view/t3jGzm

and optional reference for waveform simple: https://www.shadertoy.com/view/wX23Rm

[raysan5]

@meisei4 Thanks for the addition, definitely a great example despite a bit long... I will review it on my side...