Add support for a third color in face definition that controls
the underline and a 'c' attribute for curly underline (that takes
precedence over 'u' if both are specified)
Allow empty colors to mean default, so that `,,red+u` means the
same as `default,default,red+u`
Fixes#4138
Using <fg>,<bg>+<attr>@<base> will apply the given fg color,
bg color and attributes on top of base dynamically. Simply giving
<base> is a shorthand for default,default@<base>.
Inspired by the discussion in #2862
Also, moved the "Markup strings" section to faces.asciidoc, since it wasn't
anything to do with expansions, and updated various hyperlinks to point at the
new location.
Final is more granular, it consists of FinalFg (f), FinalBg (g)
and FinalAttr (a) which control if a face's fg, bg, or attributes
fully overwrite the previous face (instead of merging) and if
following faces apply on top of this face or not.
Fixes#2388 if the Whitespace face has the FinalFg flag.
When on an end of line, certain behaviours can be surprising, for
example delete will join the following line (which makes sense, and
is consistent, but hard to predict if we do not know the cursor is
on and end of line).
As Kakoune is moving more and more towards treating end of lines
as any other character, making it clear when the cursor lies on
them seems like a good way to reduce surprise.
