Moves this within a line, to a new character (not byte) offset.
The given character offset must be less than or equal to the number of characters in the line; if equal, this moves to the start of the next line. See [[email protected]_line_index] if you have a byte index rather than a character offset.
| this |
a `GtkTextIter` |
| char_on_line |
a character offset relative to the start of this’s current line |