[Serial] Clear traceback if the user interacts with the REPL

[microbit-matt-hillsdon]

trafficstars

We should clear the traceback shown in the compact serial bar if the user interacts with the REPL.

Currently we use the last traceback in the latest 4k of serial output.

If we do this then we can probably simplify to say it must be at the end of the traceback output (though the REPL prompt could follow it.