Add read/write breakpoints to debugger

[ZornsLemma]

In b-em it's possible to set a breakpoint on read/write to a memory address (using breakr/breakw). Is this possible in BeebEm, and if not could it be added please? The closest I can see is watching a memory address, but that doesn't interrupt execution when a read or write occurs to that address.

[richard-broadhurst]

I have this locally and would suggest that you add the option to the write breakpoint to only break when the value changes.