Markdown text in the format "**“bold text”**" cannot parse a strong syntax tree

[littltzhi]

ag：人体内的**“精、气、神”**高度合一后产生的一种生命能量。
parsed：
-document

• paragraph
  • text

[QuietMisdreavus]

This looks like the same issue as commonmark/commonmark-spec#650, which we're solving here in swiftlang/swift-cmark#78.