Content Model: Advanced cache

[JiuqingSong]

Introduce advanced cache in Content Model Editor

When edit, there will be a lot of features need to know current editing state, such as the text content before cursor, if it is editing under a table, or a list, ...

Before, we access DOM tree to retrieve these informations. Now with Content Model, we can retrieve these information better and more accurate. However, that means we need to have a up-to-date Content Model when edit. We should not always recreate Content Model every time user type something or even more the focus. So this change is to dynamically update the cached Content Model when need.

• When user just keep typing, update current focused segment(s)
• When move focus, update the selected segment(s)
• When use format API, cache the latest Content Model from ContentModelContentChangedEvent
• If we can't get enough info to update Content Model, we call still clear cache so we can recreate the Content Model according to current document

To achieve this goal, in this change I have added the following code:

1. ContentModelDomIndeser: It handles the bidirectional update by saving some info into DOM node. So that when put focus onto a node, we can easily get the realted Content Model segment(s) or table, then do porper update
2. ContentModelCachePlugin: It handles browser events and update model and DOM node using ContentModelDomIndexer
3. Put domIndexer into EditorContext, so that Content Model processor and handlers will use it to update DOM node. If it is not passed (Out-of-editor scenario, such as copy/paste, initial rewrite), we just ignore the indexer.
4. A new ExperimentalFeature: ReusableContentModelV2