Fix bug where selections always include an extra character

[MattIrv]

Our line selection logic was wrong, meaning that every selection included an extra character unless it was already at the end of a line.

Closes Microsoft/carbon#1379. See that issue for more details.

[msftclas]

@MattIrv,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[benrr101]

Well dang. I thought I got that all working correctly.
This code was a mess to begin with since it took the 0-indexed positions, converted them to 1-indexed, then back to 0-indexed to do string manipulation. I ripped out the 1-indexed stuff but it still needed some off-by-1 fixes b/c of how python does range indexing.

Good catch. Thank you for fixing my mistakes 😉

[MattIrv]

Automatically kicked off a Jenkins build for your branch fix/selectionExtraCharacter

1d5f889: http://xplat-build-vm:8080/job/pgtoolsservice_custom-branch/175/
51e4317: http://xplat-build-vm:8080/job/pgtoolsservice_custom-branch/177/

Build status: success
Tests passing: 283
Coverage: 92.55%