Content Model Cache improvement - Step 8: Finally enable readonly types in all format API

[JiuqingSong]

Now all format API are using readonly types, we can finally enable the readonly type from root level.
This includes:

1. In formatContentModel, we provide a ShallowMutableContentModelDocument to callback instead of original ContentModelDocument. Then if we need to do any modification to blocks under this model object, we need to use mutateBlock to convert it to a mutable block. This is a breaking change. Ideally after this change we should bump the major version. However, given that we just bump to version 9 recently, I'll prefer keep current major version and later we can provide a document on how to fix the build failures when integrate the new version.
2. Provide a new editor option experimentalFeatures to allow pass in feature names.
3. Provide a new feature name PersistCache. When this feature name is passed into EditorOptions.experimentalFeatures, we will keep cached element in content model as long as we can
4. In iterateSelection, do not delete cached element for selected blocks when PersistCache is enabled
5. In domIndexer, do not delete cached element for current block when PersistCache is enabled. I also need to convert it to a class so that it can hold private members
6. In demo site, add the UI to allow turn on/off experimental features.

In order to let iterateSelections know if PersistCache is enabled, I did a workaround here to store this information in Content Model as a private member. So that we don't need to change the caller code everywhere. Once this feature is fully graduated, we can remove this private member.