Normally, assigning to e.g document.body.onload will forward to
window.onload. However, in a detached DOM tree, there is no associated
window, so we have nowhere to forward to, making this a no-op.
The bulk of this change is making Document::window() return a nullable
pointer, as documents created by DOMParser or DOMImplementation do not
have an associated window object, and so must be able to return null
from here.
With this change, we now have ~1200 CellAllocators across both LibJS and
LibWeb in a normal WebContent instance.
This gives us a minimum heap size of 4.7 MiB in the scenario where we
only have one cell allocated per type. Of course, in practice there will
be many more of each type, so the effective overhead is quite a bit
smaller than that in practice.
I left a few types unconverted to this mechanism because I got tired of
doing this. :^)
