True during a session for an object that contains the end of the text so far: the last leaf block
and every block around it, those whose span ends at or after ProvisionalStart; an
inline is judged by its leaf block (ContainerInline.ParentBlock), and is never provisional outside one.
Always false when IsStreaming is false, so renderers can call it unconditionally.
public bool IsProvisional(MarkdownObject obj)