Simplify erc-button movement commands

Simplify erc-button movement commands

* etc/ERC-NEWS: Mention TAB being bound to new command `erc-tab' and
`erc-previous-button' now stopping at the start of buttons.
* lisp/erc/erc-button.el (erc-button-mode, erc-button-enable,
erc-button-disable): Add and remove `erc-button-next' to
`erc--tab-functions' hook, which is tantamount to binding the command
in the read-only area of an ERC buffer.
(erc-button-next-function): Deprecate and remove from client code path
because this module doesn't concern itself with prompt input and thus
no longer needs to conform to the `completion-at-point-functions'
interface.
(erc-button--prev-next-predicate-functions): New variable, a hook to
determine whether to continue searching for a button.  Other modules
should utilize this as needed.
(erc-button--end-of-button-p): Add function to serve as default value
for `erc-button--continue-predicate'.
(erc--button-next): Add generalized button-movement function.
(erc-button-next, erc-button-previous): Make `erc-button-previous'
behave more predictably by having it land at the beginning of buttons.
And remove roundabout appeal to HOF in `erc-button-next'.
(erc-button-previous-of-nick): New command to jump to previous
appearance of nick at point.
* lisp/erc/erc-fill.el (erc-fill-wrap, erc-fill-wrap-enable,
erc-fill-wrap-disable): Add and remove merge-related hookee from
`erc-button--prev-next-predicate-functions'.
(erc-fill--wrap-merged-button-p): New function to detect redundant
speakers.
* lisp/erc/erc.el (erc-complete-functions): Quote TAB in doc string.
(erc-mode-map): Bind `erc-tab' to TAB.
(erc--tab-functions, erc-tab): Add new command and hook to serve as
unified dispatch for TAB-related operations.  It calls `c-a-p' in the
input area and defers to module code in the read-only message area.
* test/lisp/erc/erc-button-tests.el: New file.
* test/lisp/erc/erc-fill-tests.el (erc-fill-tests--wrap-populate): Run
finalizer for transient keymap timer.
* test/lisp/erc/erc-tests.el
(erc-button--display-error-notice-with-keys): Move to new dedicated
test file for erc-button and fix expected behavior of
`erc-button-previous'.  (Bug#62834)