add pwm example

examples/flipperzero_pwm_test.py

@@ -0,0 +1,7 @@
+import flipperzero as f0
+import time
+
+f0.pwm_start(f0.GPIO_PIN_PA7, 4, 50)
+time.sleep(5)
+f0.pwm_start(f0.GPIO_PIN_PA7, 1, 50)
+time.sleep(5)