The source position where the block that contains the end of the text starts (the deepest last
block of the document), or when no block does: outside a session, on
the final update, or when a blank line follows the last block. Every block whose span ends at or
after it contains the end of the text. Read by IsProvisional(MarkdownObject) while
IsStreaming is true.
public int ProvisionalStart { get; }