nrf5/modules/music: Update modmusic to use updated includes. Add extern ticks. Add function which implements initialization of pwm and ticker, register ticker callback, and start the pwm and ticker. This corresponds to microbit port main.cpp init.
nrf5/modules/music: Update modmusic to use updated includes. Add extern ticks. Add function which implements initialization of pwm and ticker, register ticker callback, and start the pwm and ticker. This corresponds to microbit port main.cpp init.

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