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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