#2878 Provide a callback function to allow fixup model before write back

[JiuqingSong]

Some feature has requirement that content model should satisfy some format, such as there must be an entity before / after something. We now provide a callback to allow them do those fix up work before write back to editor. To use it:

const editor = new Editor(div, {
 ...
 onFixUpModel: model => {
    ... // fix up model, for example
    model.blocks.push({ ... });
  }
});