Before this change, Agent held on to all of the live MutationObserver
objects via GC::Root. This prevented them from ever getting
garbage-collected.
Instead of roots, we now use a simple IntrusiveList and remove them
from it in the finalizer for MutationObserver.
This fixes a massive GC leak on Speedometer.
We do not fire `beforeinput` events since other browsers do not seem to
do so either.
The spec asks us to check whether a command's action modified the DOM
tree. This means adding or removing nodes and attributes, or changing
character data anywhere in the tree. We have
`Document::dom_tree_version()` for node updates, but for character data
a new version number is introduced that allows us to easily keep track
of any text changes in the entire tree.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
