Add command to refill buffer in erc-fill-wrap-mode

Add command to refill buffer in erc-fill-wrap-mode

* lisp/erc/erc-fill.el (erc-fill-function, erc-fill-wrap-mode):
Mention new command `erc-fill-wrap-refill-buffer' in doc string.
(erc-fill--wrap-rejigger-last-message): New internal variable.
(erc-fill--wrap-rejigger-region, erc-fill-wrap-refill-buffer): New
command and helper function for fixing alignment issues that arise,
for example, from adjusting pixel-display widths of buffer text during
a session.
* test/lisp/erc/erc-fill-tests.el (erc-fill-tests--simulate-refill):
New function for approximating `erc-fill-wrap-refill-buffer'.
(erc-fill-wrap--merge): Assert refilling is idempotent.  (Bug#60936)