Add -indent option to show-whitespace highlighter

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

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