LibWeb: Implement document.domain getter

The document.domain setter is currently stubbed as that is a doozy to
implement, given how much restrictions there are in place to try and
prevent use of it and potential multi-process implications.

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Userland/Libraries/LibWeb/DOM/Document.h

@@ -353,6 +353,9 @@ public:
     bool is_initial_about_blank() const { return m_is_initial_about_blank; }
     void set_is_initial_about_blank(bool b) { m_is_initial_about_blank = b; }
 
+    String domain() const;
+    void set_domain(String const& domain);
+
 protected:
     virtual void visit_edges(Cell::Visitor&) override;