Add Mardal-Tai-Winther element

[DefElement page](https://defelement.org/elements/mardal-tai-winther.html); `src/mardal_tai_winther.jl` has the `STATUS: BLOCKED` notes.

H(div) Stokes element with two normal moments and one tangential moment per edge. A complete reference implementation exists (commented out; passes the symfem cross-check and pointwise normal continuity), but the tangential moments become metric-weighted under the contravariant Piola map, so neighbors with different Jacobians couple different physical functionals — verified numerically (jump vanishes only for congruent translate cells).

Blocked by #2.

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