Adds an ELResolver to the chain of EL variable and property management
within JSP pages and Tag files.
JSP has a default set of ELResolvers to chain for all EL evaluation:
ImplicitObjectELResolver
ELResolver instances registered with this method
MapELResolver
ListELResolver
ArrayELResolver
BeanELResolver
ScopedAttributeELResolver
Parameters: resolver - an additional resolver throws: IllegalStateException - if called after the application's ServletContextListeners have been initialized.