Add Continuous Rotation Option to the servo

[dela9116]

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I wish the servo device had an option for continuous rotation The PWM input controls arm rotation direction and speed instead of arm angle. These types of servos are not as common, but they are commercially available. This should be a very simple modification.

Desired behavior: Input PWM Servo behavior 0 fullspeed backward 45 halfspeed backward 90 stop 135 halfspeed forward 180 fullspeed forward X (X - 90)/ 180 * fullspeed