feat: support extending JSONFormatter

[atsjo]

Support extending JSONFormatter with custom text diff operations, and fix a potential bug where MovedDelta is identified as a TextDiffDelta:

  if (itemDelta[2] === 3) {
    throw new Error(
      "JSONPatch (RFC 6902) doesn't support text diffs, disable textDiff option",
    );
  }

As of now I have to make my own copy of JSONFormatter to extend it. By accepting this PR this would be a cleaner integration for me, and give other the same easy way of supporting custom text diffing operations by extending JSONFormatter like this:

class MyJSONFormatter extends JSONFormater {
  protected override processTextDiff(ops: Op[], path: string, diff: string) {
    const lines = parseTextDiff(diff);
    for (const line of lines) ...
  }
}