Fix trigger reconcilation on nested read

[zurfyx]

Problem

A combination of nested updates + reads can cause a missed reconcilation, consequently leading to further errors such as https://lexical.dev/docs/error?code=75&v=6 or mutating (should've been frozen) nodes.

For example:

1. dispatchCommand(KEY_DOWN)
2. (dispatch command triggers an update by command design)
3. User triggers a function such as submit message that contains a combination of editor.read and editor.update
4. editor.read flushes by design
5. editor.update has no pendingEditorState as it has been flushed. Hence, reconcilation never happens.

Solution

We can either recreate the EditorState or ignore the flush sync behavior on read. Because we already support flushSync flag in nested updates this PR does the first.

Note for self

We can't just reuse the discrete functionality within update as in

read<T>(callbackFn: () => T): T {
    const pendingEditorState = this._pendingEditorState;
    if (pendingEditorState !== null) {
      this.update(() => {}, {discrete: true});
    }
    return this.getEditorState().read(callbackFn, {editor: this});
  }

This will trigger a (parent) update and dismiss the concept of queued updates.

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[etrepum]

Related #7709