fix(lexer): Only allow horizontal whitespace in frontmatter
In writing up the reference for frontmatter, I realized that we probably
shouldn't be accepting Unicode Line Ending characters between the code
fence and infostring or trailing after the infostring or a code fence.
In digging into the unicode specification we use for Whitespace, it
divides it up into categories, so I'm deferring to what it says for
horizontal whitespace for what should be used within a line.
Note, I am leaving out support for Unicode Default Ignorable characters.
I figure that can be discussed outside of this change within the
reference and tracking issue.
Fixesrust-lang/rust#145971
Frontmatter tracking issue: rust-lang/rust#136889
56213a553e