Add direct serendipity element

[DefElement page](https://defelement.org/elements/direct-serendipity.html); `src/direct_serendipity.jl` has the `STATUS: BLOCKED` notes.

The shape functions are rational functions constructed directly on the *physical* quadrilateral (they depend on the vertex positions to retain full approximation order on non-parallelogram cells), so there is no reference-basis + mapping factorization. Not fixable by an upstream Ferrite mapping feature in the current model; would need per-cell basis construction at reinit! time.

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