Make ERC's format catalogs more extensible

Make ERC's format catalogs more extensible

* lisp/erc/erc-common.el (erc--define-catalog): Accept a `:parent'
keyword to allow for extending an existing catalog by overriding some
subset of defined entries.
(erc-define-message-format-catalog): Add edebug spec.
* lisp/erc/erc.el (erc-retrieve-catalog-entry): Check parent for
definition before looking to `default-toplevel-value'.
* test/lisp/erc/erc-tests.el (erc-retrieve-catalog-entry): Add test
case for inheritance.
* test/lisp/erc/resources/erc-tests-common.el
(erc-tests-common-pp-propertized-parts): Fix bug in convenience
command.  (Bug#67677)