add GfmBacktickSingle / GfmBacktickDouble for GFM inline code spans

Two new inline formatting modes covering GFM code spans in their n=1
and n=2 forms:

GfmBacktickSingle `text` → <code>text<

add GfmBacktickSingle / GfmBacktickDouble for GFM inline code spans

Two new inline formatting modes covering GFM code spans in their n=1
and n=2 forms:

GfmBacktickSingle `text` → <code>text</code>
GfmBacktickDouble ``text`` → <code>text</code>

Both emit monospace_open and monospace_close around an unformatted()
call (the same instruction shape as DokuWiki's two-single-quote pair
wrapping a nowiki span), so renderers that distinguish verbatim text
from plain cdata — metadata, indexer, non-XHTML backends — treat the
body as literal.

GfmBacktickDouble extends GfmBacktickSingle to reuse handle() and the
body-normalization helper; only the delimiter length and the body
character class differ. Both share sort 165 and gate on Markdown
being loaded.

Design notes:

* The lexer has no backreferences, so each length is its own mode.
Length-boundary guards (?<!`)...(?!`) on every opener and closer
ensure a run of two-or-more backticks is never read as an n=1
delimiter and a run of three-or-more is never read as n=2. The two
modes never steal each other's input regardless of registration
order — sort can't reach this kind of cross-position constraint.

* Edge-whitespace handling and newline normalization live in handle(),
not in the regex. On DOKU_LEXER_UNMATCHED the body is normalized:
1. CR/LF and LF become single spaces (GFM line-ending rule).
2. If the body starts and ends with a space and is not entirely
whitespace, one space is stripped from each end.
That produces the right GFM output for the tricky cases without
special-casing the entry pattern:
` ` → <code> </code> (all-whitespace, no strip)
` a` → <code> a</code> (asymmetric, no strip)
` `` ` → <code>``</code> (interior run-of-2 + strip)
``foo`bar`` → <code>foo`bar</code>

* Body character classes admit exactly the runs that cannot be valid
closers for this mode's length: n=1 allows `[^`] | ``+`, n=2 allows
`[^`] | `(?!`)`. That is what lets a single-backtick span contain
a pair and a double-backtick span contain a lone backtick.

* allowedModes is empty — no other inline parsing runs inside a span.

Deliberately not implemented, with skip.php entries explaining why:

351 — code-span precedence over emphasis (*foo`*` expected to render
as *foo<code>*</code>). Cross-positional: the single-pass
lexer matches leftmost-first and cannot reject an earlier
emphasis opener because a later backtick span would consume
its closer. A proper fix would need a pre-scan pass; sort
values only break ties at the same position.
353 — the trailing " outside the code span gets converted to a
curly quote by DokuWiki typography, diverging from spec HTML.
354 — raw HTML tag pass-through; DokuWiki does not render raw HTML
by default.
356 — GFM angle-bracket autolink <http://…>: not implemented.

Per-mode unit tests cover basic matching, flanking via the length-
boundary guards, interior-run support in the body, edge-space
stripping, newline normalization, all-whitespace bodies, paragraph-
boundary rejection, content-is-literal, and sort values.
ModeRegistryTest's gating data provider picks up both modes.

Net effect on GfmSpecTest: eleven previously-red code-span examples
now pass (339, 340, 341, 342, 344, 345, 346, 347, 349, 350, 357, 359
— the simple pairs, edge-space, interior-run, newline-normalization,
and mismatched-run cases). Four skipped. Three remain pending outside
the code-span scope (emphasis interactions that need GfmLink once
that lands).

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