The indentation is drawn as virtual text and the icon was only ever used
for its display width, which had to be kept in sync by hand with the
width of the expander icons. Take a number of columns instead.

BREAKING CHANGE: discussion_tree.expanders.indentation is replaced by
discussion_tree.indent_width. :checkhealth reports the removed setting.

feat: draw the expander as virtual text

The whole indent region is virtual now, so a buffer line holds nothing
but the note's own text: yanking a header no longer picks up the
expander icon and the cursor cannot sit on it. A node's text starts at
depth * width, with the expander drawn inside the node's own level,
which keeps headers aligned with the bodies and replies under them
whatever the expander icons are. A level grows to fit an expander wider
than indent_width.

feat: add indent guides to the discussion tree

Guides tie a discussion's replies to the note that started it, with the
last reply closing the thread, so that separate discussions can be told
apart at a glance. They are drawn as part of the virtual indentation and
so cost no columns and never enter the buffer. Only a discussion's root
note produces them: a reply's body is not joined to the reply's own
header. Set discussion_tree.indent_guides to false to turn off.

fixup: only insert indents on wrapped lines when 'wrap' is on