extract Helpers\HtmlEntity, wire into GfmCode and GfmLink URL slot

Numeric and named HTML entity decoding moves out of GfmHtmlEntity into
a pure helper, so capture-by-regex modes can apply the same

extract Helpers\HtmlEntity, wire into GfmCode and GfmLink URL slot

Numeric and named HTML entity decoding moves out of GfmHtmlEntity into
a pure helper, so capture-by-regex modes can apply the same decode
post-extraction (the inline lexer never reaches their bodies). Mirrors
the Helpers\Escape pattern.

Wired up in two slots:

- GfmCode info string: föö now decodes to föö in the
language class. Clears spec example #330.

- GfmLink URL: GfmLink::extractUrl() decodes entities. URL pattern
extends from `[^)\n]+` to `(?:\\.|[^)\n])+` so an escaped \) no
longer terminates the URL early; the existing post-classify
Escape::unescapeBackslashes call strips the backslashes after
Link::classify has done its work. Clears #504, #506, #508.

Skip #328 with a self-contained title-slot reason: the URL side now
decodes correctly, but the title attribute is still discarded
(DokuWiki link instructions have no title slot).

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