Function to detect inside the onNote callback function if a note is held or not.
isNoteHeld is specific to the onNote callback function. Calling this function inside other callback functions is not permitted.
Available in: Processor.
true if onNote has received a note-on event, and
false if onNote has received a corresponding note-off event. The note events must come from outside the script, e.g., from the host software or another MIDI module.