Warn of future breaking change to erc-response.tags

Warn of future breaking change to erc-response.tags

* lisp/erc/erc-backend.el (erc-parse-tags-format): New option to
determine type of the `erc-response' "tags" field.
(erc-parse-tags): Defer to internal generic function.
(erc--parse-tags): New function to hold original `erc-parse-tags'
implementation.
(erc--parse-message-tags): New generic function that conditionally
calls `erc--parse-tags', perhaps emitting a warning beforehand.
(erc-parse-server-response): Call `erc--parse-message-tags'.
(Bug#58797.)