Add hw-pwm gem with pulse width modulation support:
- Common Ruby API with frequency, duty, period_us, pulse_width_us
- ports/esp32/ using LEDC peripheral
- ports/rp2040/ using Pico SDK PWM hardware
Based on picoruby-pwm by HASUMI Hitoshi.
Co-authored-by: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto