Feature: user-set variables / system-level variables improvements

[nnirror]

• When a user sets a variable via .set(), remove any previously-set variables with same name.
• Separate into two separate files the "user-set" variables (which can potentially be expensive, containing multiple seconds of audio) from the system-level variables (which should be updated very frequently, e.g. every ~20ms): env.js and patterns.js.
• patterns.js would update as part of the callback in pattern_generator:js:run(), when this condition is true: if ( fp.variables_to_set.length > 0 ) {
• make sure all places that env.js is loaded now load both files

[nnirror]

Also, before writing env.js, check if any variables have actually changed before writing to prevent useless CPU overhead

[nnirror]

Added in: 9b21c3e