Some plugins (*cough* kak-lsp) and help texts tend to have immensely long content
in a single line. This generates info boxes that span the entire terminal width.
This is made especially worse on widescreen monitors or at small text size.
This grants user control over how wide these boxes are.
I deliberately avoid pushing this change to `kak-lsp` because it's not the only
plugin that this could help--see the `hook` help text for an example of this
problem in vanilla Kakoune. I would also suggest that since this is a rendering
concern, it be handled by the terminal rendering logic.
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