MarkupParser.parse()method

Parse a text string as Markdownish markup syntax and render it as a React node.

• Syntax is not defined by this code, but by the rules supplied to it.
• Untrusted input: parsing is linear for normal content, but a few rules can still degrade on adversarial
  input — long unbroken runs of backtick code fences, and deeply-nested > blockquotes (each leading >
  recurses one level, so a pathological line can overflow the stack). When input is user-generated, cap its
  length first; a sane maximum for your use case (for typical user content, tens of kilobytes) bounds worst-case work.