Improve accessibility line range calculation and add support for setting insertion line

Refactors accessibilityRange(forLine:) to use visual layout fragments rather than
semantic text elements, providing a more accurate mapping between visual line
numbers and text ranges. Also adds an implementation for
setAccessibilityInsertionPointLineNumber to allow screen readers or
accessibility clients to move the cursor to a specified line number.
Trailing newlines are now included in line ranges when appropriate.