Use modern fallback for channel name detection in ERC

Use modern fallback for channel name detection in ERC

* lisp/erc/erc-backend.el (erc-query-buffer-p): Remove forward declaration.
* lisp/erc/erc.el (erc-query-buffer-p): Defer to `erc-channel-p'.
(erc-channel-p): Refactor and use `erc--fallback-channel-prefixes' for
the default CHANTYPES value.  Honor an empty CHANTYPES value as valid,
e.g., for servers that only support direct messages.
(erc--fallback-channel-prefixes): New variable to hold fallback
CHANTYPES prefixes recommended by RFC1459 and modern authorities on
the matter.
* test/lisp/erc/erc-tests.el (erc-channel-p): Revise test.  (Bug#67220)