Add Stepper motor class

[pjbRPF]

Core Implementation:
• Complete Stepper class supporting 4-pin stepper motors (28BYJ-48 + ULN2003)
• Three step sequences: wave, full, half-step for different torque/smoothness needs
• Position tracking with step counting and angle calculation
• Configurable steps-per-revolution and step delays for different motors

API Design Philosophy - Four Complementary Approaches:

1. DEFAULT DIRECTION (Simplest)
   • step(10) - Uses default clockwise direction
   • Minimizes cognitive load for basic use cases

2. CONVENIENCE METHODS (Explicit Intent)
   • step_clockwise(10), step_counterclockwise(10)
   • rotate_clockwise(90), rotate_counterclockwise(90)
   • revolve_clockwise(2), revolve_counterclockwise(2)
   • Short aliases: step_cw(), rotate_ccw(), revolve_cw()
   • Clear, readable, self-documenting code

3. PARAMETERIZED METHODS (Flexible Control)
   • step(10, direction='cw'|'ccw'|1|-1)
   • rotate(90, direction='clockwise'|'counter-clockwise')
   • revolution(2, direction=1|-1)
   • Supports both string and numeric direction parameters
   • Flexible _normalize_direction() handles multiple formats

4. ALIAS SUPPORT (Compatibility)
   • revolve() as alias for revolution()
   • StepperMotor as backward-compatible class alias
   • Maintains consistency with existing picozero patterns

Technical Features:
• Polymorphic direction parameters (string/numeric) with validation
• Real-time speed control via step_delay property
• Position reset capability for homing/calibration
• Proper resource management with off() and close() methods
• Comprehensive error handling with descriptive messages

Testing Strategy:
• Validates all API approaches for consistency
• Covers edge cases and error conditions

Documentation:
• Added Stepper class to API documentation
• Exported in init.py for public access
• Comprehensive docstrings with parameter details
• Usage examples showing all API patterns

[MarcScott]

1. For current rotate method:

• Change name to turn
• Force positional argument of direction of turn (cw/ccw).

1. For current revolution method:

• Change name to rotate
• Force positional arguments for number of rotations and direction.

1. Remove methods for revolving and rotating in a specific direction
2. stepper.angle should return an angle between 0 and 359
3. Additional method rotate_to with positional arguments of angle and direction
4. Additional method step_to with positional argument of steps and direction
5. Additional method set_speed with positional argument of rotations per minute
6. Additional method run_continuous with positional argument of seconds and defaulting to running until stopped.