support waveformN format

[felixroos]

it would be nice to have waveforms like sine8 sawtooth16 etc. of the format waveformN where waveform is one of sine, triangle, sawtooth, square and N a number between 1 and 128, specifying the number of harmonics used. It is a quick and handy way to get a filtered waveform without actually needing a filter, because it's additive synthesis. This can probably with a PeriodicWave

[felixroos]

here's a codesandbox that implements the basic idea: https://codesandbox.io/s/waveformn-createperiodicwave-www4m9?file=/src/index.mjs