Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)

Summary Type: Algorithmic-complexity denial of service. A run of N closed pairs xx... or the analogous ==x== for mark, ^^x^^ for insert causes ON² work in the formatting parser. With the strikethrough, mark, or insert plugin enabled, an 8 KB input pegs the CPU for 4 seconds; 16 KB → 17 seconds...