This will probably end up being a reasonably significant refactor.

The most important requirement is to move view-handling to happen in LiteParsed; then, when the LiteParsedUnionWith is being promoted to DocumentSourceUnionWith, the subpipeline should follow suit. (Ideally, DocumentSource should have no reference to LiteParsed)

We could take it a step further to implement fully-fledged StageParams for $unionWith so that most of $unionWith parsing happens at LiteParsed::parse() and validation can happen in LiteParsed::validate().