Issue #3620811: Add the taxonomy term list base class

MyrestTermListResourceBase answers with a whole vocabulary, or a subtree of one, in the order an editor arranged it: read in one query, filtered by access and by the published state, translated, ordered, nested, rendered through the structure of the vocabulary. A subclass declares the structure, the depth, whether the answer nests, and the breadcrumb trail, and all four default to the model.

It is a class of its own rather than the listing base with a flag, and the reason is the pagination. A front end renders a vocabulary whole, and a tree cut into pages of twelve is not a smaller tree but a piece of one with the parents of the second page missing. So limit and page are not quietly ignored here - they are not offered, and the OpenAPI of the endpoint says so.

A flat answer is the tree flattened rather than the vocabulary listed in one pass: listing in one pass orders every term by its own weight, so the children of one parent land between the children of another, and the subtree and the depth a request asked for have nowhere to apply.

The other half of the pair is deliberately absent. A term detail endpoint has no base class of its own, because a term is a content entity like any other and MyrestEntityDetailResourceBase serves one from a subclass with an empty body. The fixture module carries such a subclass so that this is tested rather than asserted.

Closes #3620811