add GfmEscape for GFM backslash escapes

Implements GFM §6.1 backslash-escape handling. GfmEscape is a sort-5
inline mode in CATEGORY_SUBSTITION that claims `\X` for any escapable
ASCII punctuation c

add GfmEscape for GFM backslash escapes

Implements GFM §6.1 backslash-escape handling. GfmEscape is a sort-5
inline mode in CATEGORY_SUBSTITION that claims `\X` for any escapable
ASCII punctuation char before competing delimiters can match. The
shared character class lives on Helpers\Escape so the lexer pattern
and the post-hoc unescape stay in lockstep.

Whole-span captures (GfmCode info string, GfmLink label/URL) bypass
the lexer; those modes call Escape::unescapeBackslashes() on the
relevant slot. GfmLink skips the unescape when the URL classifies as
a windowssharelink so the leading \\host survives intact.

GfmTable cells get a separate per-cell `\|` to `|` pass in the
rewriter to honour the tables-extension rule that pipes always
unescape, even inside code spans where standard §6.1 escapes don't
fire.

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