Add the taxonomy term list and term detail base classes

Problem/Motivation

Vocabulary listings differ from entity listings in what the front end needs from them: the hierarchy, usually the whole tree at once, ordered by weight rather than paginated. Serving them through the generic listing class would make every consumer paginate a tree.

Proposed resolution

• MyrestTermListResourceBase: whole vocabulary or a subtree, in weight order, with depth, honouring access and translation.
• Term detail through the entity detail base class, with no separate class — a term is a content entity like any other once the base classes stop assuming a node.
• Cache tags per vocabulary so that editing one term invalidates the tree.

Remaining tasks

Everything.