Fixes #11848 Moving line up without line ending

[jamsinclair]

Bug

Fixes the bug described in issue #11848

Fix

From what I explored it looks like a previous dev has tried to fix this bug. Although because rows are 0 indexed, to select the penultimate row for the range it requires using -2.

I wonder whether it's worth rewriting the line:

lines += '\n' unless lines[lines.length - 1] is '\n'

I've attempted to add a valid test. Suggestions to improve or better represent the test welcome 😸

[borishere]

Just why this is not yet merged??

[lee-dohm]

Thanks for this!