add some extmark-edge-case tests from morph.nvim

lua/u.lua

@@ -360,7 +360,7 @@ function Extmark.from_range(range, nsid)
   local r = range:to_charwise()
   local stop = r.stop or r.start
   local end_row = stop.lnum - 1
-  local end_col = stop.col
+  local end_col = r:is_empty() and (r.start.col - 1) or stop.col
   if range.mode == 'V' then
     end_row = end_row + 1
     end_col = 0
@@ -702,7 +702,7 @@ end
 function Range:to_charwise()
   local r = self:clone()
   r.mode = 'v'
-  if r.stop:is_col_max() then r.stop = r.stop:as_real() end
+  if r.stop and r.stop:is_col_max() then r.stop = r.stop:as_real() end
   return r
 end