use cache to store char-to-column mapping

This is faster by O(n) to O(n²) compared to the previous algorithm
that would re-compute the column for each call.

highlight_selection.py

@@ -45,7 +45,10 @@ class HighlightSelection(Highlight):
 
             start_column = line.char_to_column(start_char)
             end_column = line.char_to_column(end_char)
-            length_in_columns = end_column - start_column
+            if end_column >= 0:
+                length_in_columns = end_column - start_column
+            else:
+                length_in_columns = 4096
 
             return [HighlightedRange(start_column, length_in_columns, brush=QBrush(QColor(156, 215, 255, 192)),
                                      pen=Qt.PenStyle.NoPen)]