[Serial] Add a prompt to compact serial bar to expand it when there's output

[microbit-matt-hillsdon]

Add a visible prompt to the compact serial bar to suggest the user expands it to see output if any output has been written.

Questions

Should this include the REPL prompt? That might not be that useful as it's shown when the program falls off the end.

[microbit-matt-hillsdon]

Also useful for the simulator serial, though we're more space constrained there.