Add Same as translation to the Xliff Target content options

Problem/Motivation

Be able to select Same as translation in the Xliff File provider so any existing translation can be included in the xlf file.

This is a variation of #3092262: Include existing translation content in file translator export file → that tries to change the config to
- not copy at all (current behaviour of tmgmt file)
- copy the source in any case (current behaviour of tmgmt file)
- copy the translation in any case, with no fallback to the source
- copy the translation with a fallback to the source (3092262)

Then propose an integration with content moderation #2978341: Support pending revisions and accepting translation as a specific moderation state → (should be a follow up).

Proposed resolution

Add Same as translation to the existing Empty and Same as source options,
so the current behaviour doesn't change on existing setups and plugins that are extending FileTranslator could provide a distinct configuration.

As it requires to choose between including the source or not in the initial translation, a more flexible approach could be to use checkboxes here. E.g. unchecked = Empty then Same as source and/or Same as translation. So if only Same as translation is checked it should mean that the initial translation might not be included in the source.