Add envelope generator script

[roryjamesallen]

A simple envelope generator script: din triggers an envelope (and sustains it if on asr mode) k1 controls rise time k2 controls fall time ain is added to k2 to control fall time (i'm planning to allow ain to be routed to other parameters) b1 changes mode between ar (Attack Release) and asr (Attack Sustain Release) b2 changes mode between once and looping envelopes

The envelope is displayed as linear (currently) but the actual output is logarithmic, so works well for audio

[awonak]

Don't forget to add your script to the central contrib readme! https://github.com/Allen-Synthesis/EuroPi/blob/main/software/contrib/README.md