Favor ISUPPORT params for MODE processing in ERC

Favor ISUPPORT params for MODE processing in ERC

* etc/ERC-NEWS: Mention shift toward CHANMODES ISUPPORT parameter for
dictating MODE parsing behavior.
* lisp/erc/erc-backend.el (erc--init-channel-modes, erc--update-modes,
erc-set-modes, erc-update-modes): Forward declarations, the last two
being removals.
(erc-server-MODE, erc-server-221): Use `erc--update-modes' instead of
`erc-update-modes'.
(erc-server-324): Use `erc--init-channel-modes' instead of
`erc-set-modes'.
* lisp/erc/erc-common.el (erc--channel-mode-types): New struct for
stashing processed \"CHANMODES\" data for the current server.
* lisp/erc/erc.el (erc-channel-modes): Fix doc string.
(erc-set-initial-user-mode): Display a local notice when requesting
redundant user MODE operations.
(erc-set-modes, erc-parse-modes, erc-update-modes): Deprecate for
reasons explained in associated ERC-NEWS entry.
(erc--update-membership-prefix): New function, a helper for specifying
arguments to the rather unruly `erc-update-current-channel-member'.
(erc--channel-modes): New variable to record channel-mode state in a
hash table.
(erc--channel-mode-types): New variable and getter to stash
and retrieve server-local instance of the struct of the same name.
(erc--process-channel-modes): New function to parse channel-mode
changes, dispatch handlers for unary modes, and update the local
variables `erc-channel-modes' and `erc--channel-modes'.
(erc--user-modes): New local variable for remembering user modes per
server.  New function of the same name, a "getter" for the variable.
(erc--parse-user-modes): New function to parse user modes only.
(erc--update-user-modes): New function to update and sort
`erc--user-modes'.
(erc--update-channel-modes): New function to replace much of
`erc-update-modes', currently a thin wrapper around
`erc--process-channel-modes' to ensure it updates status prefixes.
(erc--update-modes): New function to call appropriate mode-updating
function for the current buffer.
(erc--init-channel-modes): New function to update channel mode letters
without status prefixes.
(erc--handle-channel-mode): New generic function, a placeholder for an
eventual API to handle specific "unary" mode letters, meaning those
that specify a single parameter for setting or unsetting.
(erc-update-channel-limit): Update doc string and answer question
posed by ancient comment.
(erc-message-english-user-mode-redundant-add,
erc-message-english-user-mode-redundant-drop): New English catalog
messages.
* test/lisp/erc/erc-scenarios-base-chan-modes.el: New file.
* test/lisp/erc/erc-tests.el (erc-parse-modes,
erc--update-channel-modes, erc--update-user-modes, erc--user-modes,
erc--parse-user-modes): New tests.
* test/lisp/erc/resources/base/modes/chan-changed.eld: New test data
file.  (Bug#67220)