LibWeb: Make the node mutation event functions spec compliant

This particularly affects the insertion steps and the removed steps.

The insertion steps no longer take into the parent that the node
was inserted to, as per the spec. Due to this, I have renamed the
function from "inserted_into" to simply "inserted". None of the
users of the insertion steps was using it anyway.

The removed steps now take a pointer to the old parent instead of
a reference. This is because it is optional according to the spec
and old parent is null when running the removal steps for the
descendants of a node that just got removed.

This commit does not affect HTMLScriptElement as there is a bit
more to that, which is better suited for a separate commit.

Also adds in the adopted steps as they will be used later.

Userland/Libraries/LibWeb/HTML/FrameHostElement.h

@@ -45,7 +45,7 @@ public:
 
     void content_frame_did_load(Badge<FrameLoader>);
 
-    virtual void inserted_into(Node&) override;
+    virtual void inserted() override;
 
 protected:
     RefPtr<Frame> m_content_frame;