LibWeb: Refactor "editable" and "editing host" concepts

The DOM spec defines what it means for an element to be an "editing
host", and the Editing spec does the same for the "editable" concept.
Replace our `Node::is_editable()` implementation with these
spec-compliant algorithms.

An editing host is an element that has the properties to make its
contents effectively editable. Editable elements are descendants of an
editing host. Concepts like the inheritable contenteditable attribute
are propagated through the editable algorithm.

Libraries/LibWeb/HTML/FormAssociatedElement.h

@@ -170,6 +170,9 @@ public:
     bool has_scheduled_selectionchange_event() const { return m_has_scheduled_selectionchange_event; }
     void set_scheduled_selectionchange_event(bool value) { m_has_scheduled_selectionchange_event = value; }
 
+    bool is_mutable() const { return m_is_mutable; }
+    void set_is_mutable(bool is_mutable) { m_is_mutable = is_mutable; }
+
     virtual void did_edit_text_node() = 0;
 
     virtual GC::Ptr<DOM::Text> form_associated_element_to_text_node() = 0;
@@ -205,6 +208,9 @@ private:
 
     // https://w3c.github.io/selection-api/#dfn-has-scheduled-selectionchange-event
     bool m_has_scheduled_selectionchange_event { false };
+
+    // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#mutability
+    bool m_is_mutable { true };
 };
 
 }