Simplify ISUPPORT-derived data wrangling in ERC

Simplify ISUPPORT-derived data wrangling in ERC

* lisp/erc/erc-backend.el (erc--get-isupport-entry): Check server
buffer for `erc-server-parameters' when (re)initializing value.  This
function was previously unreliable from a target buffer on cache
misses.
(erc--with-isupport-data): New macro for accessing and caching data
derived from an ISUPPORT parameter.  Late-arriving params break the
cache.
(erc-server-005): Rewrite pattern as `rx' form, factoring out bol/eol.
* lisp/erc/erc-common.el (erc--isupport-data): New struct to be
subclassed for storing cached ISUPPORT-derived data.
* test/lisp/erc/erc-scenarios-display-message.el: Remove stray
`require'.  (Bug#67220)