LibJS: Move 'typeof' string functionality from AST to Value

We should be able to get the 'typeof' string for any value directly, so
this is now a standalone Value::typeof() method instead of being part of
UnaryExpression::execute().

Userland/Libraries/LibJS/Runtime/Value.h

@@ -303,6 +303,8 @@ public:
         return *this;
     }
 
+    String typeof() const;
+
 private:
     Type m_type { Type::Empty };