rewrite_reflow: Extract `split_line` function

Extract `split_line` function from `reflow_lines` to reimplement it in
the future without using SCI_SPLITLINES to achieve the
behaviour consistent with the "Line breaking" option.
rewrite_reflow: Create `sci_get_position_from_col` function to wrap Scintilla's SCI_FINDCOLUMN command.
rewrite_reflow: Reimplement `split_line` function to achieve consistency with the "Line breaking" option
rewrite_reflow: Remove no longer used `sci_lines_split` function that wrapped SCI_LINESSPLIT
rewrite_reflow: Remove a check inside line breaking implementation that caused breaking to occur too early