Cannot use COM port/Web Serial serial based plotter at the same time as the editor

[microbit-matt-hillsdon]

If you try to use a WebUSB serial plotter at the same time as the editor then they fight over the serial data:

https://sekigon-gonnoc.github.io/web-serial-plotter/

We do disconnect the editor when you change tab, but that itself is something of a workaround, see #853, #89.

[microbit-matt-hillsdon]

We'd probably need some way to explicitly disable serial in the editor.