Treat RSTRING_END return value as a char* instead of string

`RSTRING_END` expects pointer to return
https://github.com/ruby/ruby/blob/v3_4_2/include/ruby/internal/core/rstring.h#L434-L442

Author: sue445

config/default.yml.erb

@@ -81,6 +81,8 @@ function:
     <% end %>
 
   pointer_hint:
+    RSTRING_END:
+      self: raw
     RSTRING_PTR:
       self: raw
     rb_data_object_make: