At present, Adafruit's rotary encoders all move 1 quadrature cycle per
detent, so we originally hard-coded division-by-4. However, other
encoders exist, including ones without detents, ones with 2 detents per
cycle, and others with 4 detents per cycle.
The new `divisor` property and constructor argument allows selecting
a divisor of 1, 2, or 4; with the default of 4 giving backward
compatibility.
The property is not supported (yet?) on espressif MCUs; it throws an
error if a value other than 4 is set.
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