Polyfilling mbed for extensions

[microbit-sam]

Implements: PwmOut, InterruptIn, DigitalIn, DigitalOut, Ticker, (Mbed)Timer

Extensions wishing to use any of these will need to make sure that they #include "mbed.h"

Is there a define extensions can use determine which timer to use? e.g. Timer in on v1, and MbedTimer on v2

#ifdef IS_V2
MbedTimer timer;
#elseif
Timer timer;
#endif

Note: commit messages referencing extensions were chosen as they were top of this list rather than any favoritism!
microsoft/pxt-microbit#3506

@finneyj please critique!

[microbit-sam]

This doesn't compile properly in PXT..

error: reference to 'uBit' is ambiguous
   39 |             uBit.messageBus.listen(DEVICE_ID_MBED_TICKER, 0x0, this, &Ticker::onTick);
      |             ^~~~

~/pxt-microbit-next/projects/Untitled/built/codal/pxtapp/pxt.h:46:17: note: candidates are: 'codal::MicroBit pxt::uBit'
   46 | extern MicroBit uBit;

[microbit-sam]

Fixed by using __has_include to include pxt.h if it exists

I can replace it with a check for a define if __has_include isn't supported in all environments? Couldn't find a PXT_EXISTS type define anywhere

[microbit-sam]

Now uses MICROBIT_DAL_FIBER_USER_DATA to determine if it is in PXT or not

[finneyj]

Thanks Sam - this looks really good. nice job - thanks!