Migrate HTM4::ElementDescription to TypedData

[casperisfine]

Ref: #2808

Since it has no mark function, we can enable write barrier without changing any code.

cc @flavorjones @etiennebarrie

[flavorjones]

Looks good! Thanks.

(Side note: this morning I briefly looked into avoiding a T_DATA struct entirely, and just calling htmlLookupTag every time we need to access it, but that function does a binary search using string comparisons ... I didn't benchmark but it feels less efficient than just dereferencing a pointer.)