I suggest to split the task of implementing the new `extends="…"` attribute into smaller technical tickets, starting with this one here:
[] In the text, a `<ref>` with the new attribute is rendered as all others, aswith a single sequential numberub-number like "1.3".
[] In the `<references />` section, the reference is rendered as all others, but the content is a concatenated combination of both the parent and child reference, with a hard-coded space (similar to `follow="…"`).
In other words:
* Nowith indentation and sub-numbering.
* No indention[] Include parser tests if applicable.
* The order in the `<references />` section doesn't matter[] Don't deal with any edge cases.
* But since the sub-reference repeats it's parents content, it does make sense on it's own[] Don't touch VisualEditor use cases.
Depends on: T236257
This is the recent proof-of-concept, which visually accomplishes the task here: https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Cite/+/530399/