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.

show more ...