Right now, a DOMTransformer (used in HTML Copy + Paste) returns an object containing an OutlineNode and any (text) format flags information that should be applied to child TextNodes.

type DOMTransformOutput = {
  node: OutlineNode | null,
  format?: TextFormatType,
};

In addition to/instead of a format property, we should allow the DOMTransformer to return a childTransformer of type DOMTransformer that can apply any logic to the children:

type DOMTransformOutput = {
  node: OutlineNode | null,
  format?: TextFormatType,
  childTransformer?: DOMTransformer
};