Leverage DOMInsertPoint, remove posContainer/offset

[JiuqingSong]

In old editor we have a NodePosition interface which stores a node-offset pair. In new editor I removed it since we should focus on using Content Model to represent selection.

However, now I realized that in some places it is still continent to have such an interface, so I added an interface DOMInsertPoint in an earlier change. Now we can leverage this interface in several places, such as

• UndoPluginState for the auto complete insert point
• FormatPluginState for the pending format insert point

This is a refactor only, no logic change