add midi and freq unit conversions

[ogbabydiesal]

for reference

freqToMidi()
midiToFreq()

[kemkartanya]

@ogbabydiesal I can do this

[kemkartanya]

where these to be added @ogbabydiesal

[ogbabydiesal]

@kemkartanya lets chat about this here - anyone else want to weigh in? i personally think it would be super cool to have this - we want to make sure that we are only adding things which would be useful to have in a beginner friendly library and we aren't adding features which might be unnecessary. part of the redesign process was to trim the excess - but I feel like I thought of some good reasons to have this but can't remember them now haha