Parsing: make parse syntax a per-parse value, drop ModeInterface

The active parse's syntax flavour is a per-parse question, not process-
global state: within a single request a plugin can render bun

Parsing: make parse syntax a per-parse value, drop ModeInterface

The active parse's syntax flavour is a per-parse question, not process-
global state: within a single request a plugin can render bundled
DokuWiki-syntax text inside an otherwise-Markdown page. Yet ModeRegistry
was a singleton that read $conf['syntax'] and the $PARSER_MODES global,
and every mode reached it through ModeRegistry::getInstance() — so the
flavour lived in shared mutable state that two parses in one request
would fight over.

Make the registry a short-lived value instead:

- ModeRegistry is constructed once per parse with an explicit $syntax
and injected into Parser, Handler and every mode. getSyntax() /
isDwPreferred() / isMdPreferred() consult $this->syntax; the
DOKU_UNITTEST-gated mode-list cache hack is gone (each registry is
fresh, nothing to invalidate).
- p_get_instructions() is now the single place in the pipeline where
$conf['syntax'] is read; from there the flavour travels as a
parameter. No code under inc/Parsing/ reads $conf['syntax'] directly
anymore — the five syntax-reading modes (Preformatted, GfmHr,
GfmEscape, Externallink, GfmQuote) route through $this->registry.

Keep the two concepts apart, as documented in the ModeRegistry and
AbstractMode docblocks: the user's configured *preference* stays in
$conf['syntax'] for UI code (toolbar, settings), while the active
parse's syntax is a parameter carried by the registry.

$PARSER_MODES is demoted to a deprecated, read-only mirror, published
during loadPluginModes() — third-party syntax plugins (columnlist,
alphalist2, phpwikify, skipentity) and the bundled info plugin read the
global directly, often from their constructors, so the taxonomy must
stay visible there. No core code reads the mirror.

Fold ModeInterface into AbstractMode while here: getSort()/handle() are
abstract, the connect callbacks carry defaults, and the public $Lexer
"FIXME should be done by setter" becomes setLexer()/getLexer() injected
by Parser::addMode() alongside the registry. Nested-content resolution
moves to the allowedCategories()/filterAllowedModes() hooks, resolved
once when the registry is attached.

Tests build their own parser/registry through ParserTestBase::setSyntax()
instead of mutating $conf and calling the removed ModeRegistry::reset().

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rename syntax flavors 'dokuwiki' / 'markdown' to 'dw' / 'md'

Symmetry with the existing 'dw+md' / 'md+dw' setting values.

add GfmDeleted mode for GFM strikethrough (`~~text~~`)

Shares the deleted_open/deleted_close instructions with DW's <del> mode.
Entry/exit anchors `(?<!~)` / `(?!~)` reject runs of three or more til

add GfmDeleted mode for GFM strikethrough (`~~text~~`)

Shares the deleted_open/deleted_close instructions with DW's <del> mode.
Entry/exit anchors `(?<!~)` / `(?!~)` reject runs of three or more tildes
so fenced-code markers remain untouched. Also trim redundant class-level
docblocks on sibling Gfm test files.

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add GFM emphasis and underscore-delimited strong modes

Three new inline formatting modes for GitHub Flavored Markdown:

GfmEmphasis `*text*` → <em>
GfmEmphasisUnderscore `_text_`

add GFM emphasis and underscore-delimited strong modes

Three new inline formatting modes for GitHub Flavored Markdown:

GfmEmphasis `*text*` → <em>
GfmEmphasisUnderscore `_text_` → <em> (MD-preferred only)
GfmStrongUnderscore `__text__` → <strong> (MD-preferred only)

All three emit the same handler instructions as DokuWiki's Emphasis /
Strong, so existing renderers need no changes.

Design notes:

* Lexer mode names use snake_case (gfm_emphasis, gfm_emphasis_underscore,
gfm_strong_underscore) to keep PascalCase readable at the class level.
The asterisk variant emits `emphasis_open`/`emphasis_close` via the
getInstructionName() hook, so DW's Emphasis (`//...//`) and
GfmEmphasis (`*...*`) can coexist in mixed modes without a lexer
state collision while still producing the same <em> output.

* Underscore variants gate on Markdown-preferred syntax (`markdown`,
`md+dw`) because `__` otherwise means DW underline. GfmStrongUnderscore
sorts at 70 (matching Strong) — below Underline at 90 — so when loaded
it wins the lexer race for `__` runs. Underline is already gated out
of MD-preferred modes in the previous commit.

* Entry patterns enforce the simplified CommonMark flanking rules
already shared across DW inline modes (non-whitespace adjacency,
no paragraph-boundary crossing) plus the word-boundary check for
underscore variants using NO_WORD_BEFORE / NO_WORD_AFTER. The
positive non-word-char enumeration makes them multibyte-safe without
requiring the `u` flag: `für_etwas` and `пристаням_стремятся_`
correctly stay literal.

Per-mode unit tests cover basic matching, single-char bodies,
leading/trailing-whitespace rejection, empty-delimiter rejection,
paragraph-boundary rejection, multibyte intraword protection, and
sort values. ModeRegistryTest's gating data provider picks up the
three new rules.

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