Matching indent to blank lines can be confusing

[microbit-matt-hillsdon]

It's quite easy to end up with a whitespace indent on a blank line from a delete like this:

(arguably the location of our indent lines makes this misleading)

Then if you drag content to that line we match the three spaces on the blank line not the four spaces on the line above.

That results in a pile of confusing errors:

Probably originally seen by Lorna but hard to be sure this repro matches.

[microbit-matt-hillsdon]

The fix on #963 results in this end result:

So something has gone awry.

[microbit-matt-hillsdon]

Confirmed that https://github.com/microbit-foundation/python-editor-v3/pull/1037 addresses this.