Prevent left-flanking and right-flanking delimiter runs followed by multi-byte whitespace or punctuation characters

[Witiko]

The CommonMark 0.31.2 spec defines left-flanking and right-flanking delimiter runs as follows (emphasis mine):

A left-flanking delimiter run is a delimiter run that is (1) not followed by Unicode whitespace, and either (2a) not followed by a Unicode punctuation character, or (2b) followed by a Unicode punctuation character and preceded by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.

A right-flanking delimiter run is a delimiter run that is (1) not preceded by Unicode whitespace, and either (2a) not preceded by a Unicode punctuation character, or (2b) preceded by a Unicode punctuation character and followed by Unicode whitespace or a Unicode punctuation character. For purposes of this definition, the beginning and the end of the line count as Unicode whitespace.

As discussed with @lostenderman in #436 (comment) and below, we currently fail to correctly recognize following Unicode whitespace and punctuation characters whose UTF8-encoded representation is longer than one byte.

Requirements

• Add a test file that reproduces the issue.
• Fix the issue.