Read the rename rule from the parser function's own arguments

[alistair3149]

{{#batchupload:Pics|rename = /^IMG_/-->Trip-}} now sets the rename
rule. +rename keeps working, but is deprecated. The rule is taken from
the parser function's arguments rather than cut out of the file page
text in the browser, which is what needed the +.

rename= is no longer passed to the template: {{#batchupload:T|rename=x}}
used to give template T a parameter rename, and now sets the rule.

A rule that is not a pattern between delimiters, --> and a replacement
now shows a warning on the upload area, which then starts full, instead
of being left in every file page's text. A rule inside <nowiki> can
contain |. A page still served from the parser cache renames nothing,
and shows the +rename warning, until it is parsed again.
Special:BatchUpload is unaffected.

The README's renaming section is rewritten around a table of examples and a table of the
parameter's parts.

_{AI-authored — Claude Code, Opus 5.5; suggestion from @alistair3149, design settled over 2 rounds of questions, README rewrite asked for after; diff not yet human-reviewed; Vitest, PHPUnit, PHPCS and PHPStan run locally, checked in Chrome (Vector 2022, also uselang=x-xss), CI green (22 checks).}