A couple of semi-opinionated choices were made in this implementation:
1. The guide is hidden in the first column.
2. The indent guides are highlighted using a new `WhitespaceIndent` face.
3. Nothing is done to continue the guide through empty lines. I believe this to be the correct approach,
at least as long as it is kept as a part of the show-whitespaces highlighter. However some people's
oppinion may differ, and if so, that could be implemented.
4. The guides default to on, like the other show-whitespace options. Default character is "│".
5. Spaces between the indent guides are currently highlighted as other spaces.
Other reasonable options would be no replacement, -tabpad, or a similar -indentpad.
6. Guides are disabled by passing `-indent ""`.
7. Indent guides are separate from tab highlighting.
Additionally, we could consider adding a separate face for the "current" indent level as many editors do,
but this is a bit harder in kakoune because of multiple selections.
Closes#2323
b9c60e2578