float: add "solid" border style (#14310)

It looks solid with the default `FloatBorder` group.

If you set the bgcolor of FloatBorder to the same color as for FloatNormal, you
effectively get an "1-cell padding".

src/nvim/api/private/helpers.c

@@ -1765,6 +1765,7 @@ static void parse_border_style(Object style, FloatConfig *fconfig, Error *err)
     { "double", { "╔", "═", "╗", "║", "╝", "═", "╚", "║" }, false },
     { "single", { "┌", "─", "┐", "│", "┘", "─", "└", "│" }, false },
     { "shadow", { "", "", " ", " ", " ", " ", " ", "" }, true },
+    { "solid", { " ", " ", " ", " ", " ", " ", " ", " " }, false },
     { NULL, { { NUL } } , false },
   };