Mistune inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs

Summary Type: Algorithmic-complexity DoS in core emphasis parsing. A long sequence of well-formed x strong or x strong-emphasis combined pairs causes ON² parser work. Distinct from the bracket-bomb DoS repetition and from the formatting-plugin DoS /==/^^; this one fires on default-config mistune...