LibWeb: Extract changing an attribute to its own method

This will be needed outside of this the method to set an existing
attribute, so do not inline this implementation.

Userland/Libraries/LibWeb/DOM/Attr.h

@@ -33,6 +33,7 @@ public:
 
     DeprecatedString const& value() const { return m_value; }
     void set_value(DeprecatedString value);
+    void change_attribute(DeprecatedString value);
 
     Element* owner_element();
     Element const* owner_element() const;