Fix super errors line highlighting bug (#1907)

I accidentally used the last line's last highlighted char index as the
boundary for the first line's last highlighted char index. First line
should highlight all the way til the end. Also renamed things to make
this clear in the future.

Author: chenglou

jscomp/super_errors/super_misc.ml

@@ -32,7 +32,12 @@ let leading_space_count str =
   starting from the first erroring character?) *)
 (* Range coordinates all 1-indexed, like for editors. Otherwise this code
   would have way too many off-by-one errors *)
-let print_file ~is_warning ~range:((start_line, start_char), (end_line, end_char)) ~lines ppf () =
+let print_file 
+~is_warning 
+~range:((start_line, start_line_start_char), (end_line, end_line_end_char)) 
+~lines
+ppf 
+() =
   (* show 2 lines before & after the erroring lines *)
   let first_shown_line = max 1 (start_line - 2) in
   let last_shown_line = min (Array.length lines) (end_line + 2) in
@@ -103,13 +108,13 @@ let print_file ~is_warning ~range:((start_line, start_char), (end_line, end_char
       if current_line_strictly_between_start_and_end_line then
         fprintf ppf "%c@," current_char
       else if i = start_line then begin
-        if j == (start_char - columns_to_cut) then fprintf ppf highlighted_content;
+        if j == (start_line_start_char - columns_to_cut) then fprintf ppf highlighted_content;
         fprintf ppf "%c@," current_char;
-        if j == (end_char - columns_to_cut) then fprintf ppf "@}"
+        if j == current_line_cut_length then fprintf ppf "@}"
       end else if i = end_line then begin
         if j == 1 then fprintf ppf highlighted_content;
         fprintf ppf "%c@," current_char;
-        if j == (end_char - columns_to_cut) then fprintf ppf "@}"
+        if j == (end_line_end_char - columns_to_cut) then fprintf ppf "@}"
       end else
         (* normal, non-highlighted line *)
         fprintf ppf "%c@," current_char