[Toolkits] Insert code line break issue with blank editor

[microbit-matt-hillsdon]

Code in the CMS is sensible, so this is a bug in our handling of the imports:

from microbit import *
import music


display.scroll('score', delay=100, loop=True, wait=False) 
music.play(music.ODE)

Should be easy to repro in a unit test.

[microbit-matt-hillsdon]

I pushed a failing test in 736bc18.

Cause is that we prepend a newline to an inserted import line if there are no existing imports, but we fail to account for the fact we might add more than one.

A direct fix seems a bit messy. I think we could do with a better approach here that's a bit more line oriented. It is awkward that you can't see the intermediate state though.

[microbit-matt-hillsdon]

I think we should rejig so that we suffix not prefix the lines we insert. If nothing else, it'll be much more intuitive!

That means the insertion point should be the beginning of the line after the last import line (i.e. after the line break). Hopefully we can use CM line-aware API to make that jump.

That'll leave a special case where the document is a single line (and therefore has no line breaks) and that line contains an import. In that case only, we should prefix the first added import with a newline.

We should keep the code we have that additionally suffixes the last inserted import with two blank lines if all imports were newly added.