HTMLElement is the only interface that includes ElementContentEditable
in the HTML specification. This makes sense, as Element is also a base
class for elements in other specifications such as SVG,
which definitely shouldn't be editable.
Also adds a test for the attribute based on what Andreas did in the
video that added it.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
