More flexible Timer control

[gfwilliams]

The internal timer system in Espruino is pretty powerful. Right now we have it exposed in a few different ways:

• Waveform
• analogWrite(..., {forceSoft})
• digitalPulse
• Pin.writeAtTime
• Stepper
• E.dumpTimers (for debug)

But there are some things it can do that we don't expose:

• Synchronised PWM on up to 4 different pins (eg for positive+negative outputs of a square wave)
• Output a value on up to 4 different pins as a set time
• Executing a function from an interrupt: covered in Utility Timer native functions #476

... and probably more.

I'm not sure how to do this nicely, but it seems kind of crazy that we currently have (at least) 5 different APIs for using the same timer system. It might be nice to have a simple JS API that allows timers to be queried/updated and added programatically, and then we ditch E.dumpTimers and Pin.writeAtTime which nobody really uses