Account for leading timestamps in erc-match

Account for leading timestamps in erc-match

* lisp/erc/erc-match.el (erc-text-matched-hook): Mention that stamps
may be present in the narrowed buffer but absent from the message
parameter.
(erc-match--message): New function containing what was the body of
`erc-match-message' as if the latter were simply renamed.
(erc-match-message): Move body to `erc-match--message' and call it
with more aggressive narrowing.  This fixes a regression stemming from
d880a08f "Cement ordering of essential hook members in ERC".  Special
thanks to Libera.Chat user jrm for reporting this bug.  (Bug#60936)
* test/lisp/erc/erc-scenarios-match.el: New test file.