Note that these aren't full implementations of the bindings. This
mostly implements the low hanging fruit (namely, basic reflections)
There are some attributes that should be USVString instead of
DOMString. However, USVString is a slightly different definition
of DOMString, so it should suffice for now.
Instead of creating extremely common FlyStrings like "id" and "class"
on demand every time they are needed, we now have AttributeNames.h,
which provides Web::HTML::AttributeNames::{id,class_}
This avoids a bunch of string allocations during selector matching.
