This adds an initial specification of the machine.PWM class, to provide a
way to generate PWM output that is portable across the different ports.
Such functionality may already be available in one way or another (eg
through a Timer object), but because configuring PWM via a Timer is very
port-specific, and because it's a common thing to do, it's beneficial to
have a top-level construct for it.
The specification in this commit aims to provide core functionality in a
minimal way. It also somewhat matches most existing ad-hoc implementations
of machine.PWM.
See discussion in #2283 and #4237.
Signed-off-by: Damien George <damien@micropython.org>
