Two fixes for text field accessibility:
1. Fix character count being off by one due to trailing newline always
being appended to the last line. Add `is_last_line` parameter to
`accessibility_create_sub_text_edit_elements()` to control this.
2. Link adjacent TextRuns via `previous_on_line`/`next_on_line` so
screen readers can properly navigate lines. Without these links,
AccessKit treats each TextRun as a separate line, causing incorrect
announcements when arrowing through multi-line text.
