Make an distinction between Unicode whitespace and regular whitespace

[TimothyGu]

The spec makes an distinction between "whitespace" and "Unicode whitespace": whereas the latter include many additional whitespace characters, particularly the non-breaking space (U+00A0), the former does not.

Per ECMA-262 6th Edition ("ECMAScript 2015") §21.2.2.12 CharacterClassEscape, the JavaScript \s escape character matches the characters specified by "Unicode whitespace," but not "whitespace."

To fix this issue, rename the existing regular expression variable to UnicodeWhitespace, and create and use a new regular expression variable that only matches the limited set of "whitespace" characters.

The test suite does not yet cover this distinction. I will add a corresponding test case when this pull request has been accepted.

For additional information, the distinction in the spec was challenged and reaffirmed by commonmark/commonmark-spec#343.

[TimothyGu]

For the sake of completeness, I did an audit on the usage of whitespace characters in this library.

1. parseBackticks: Should use whitespace for code spans

   The contents of the code span are the characters between the two backtick strings, with leading and trailing spaces and line endings removed, and whitespace collapsed to single spaces.

2. scanDelims: Should use Unicode whitespace for delimiter runs:

   A left-flanking delimiter run is a delimiter run that is (a) not followed by Unicode whitespace, ...

   A right-flanking delimiter run is a delimiter run that is (a) not preceded by Unicode whitespace, ...

3. parseCloseBracket: Should use whitespace for inline link:

   An inline link consists of a link text followed immediately by a left parenthesis (, optional whitespace, an optional link destination, an optional link title separated from the link destination by whitespace, optional whitespace, and a right parenthesis ).

This pull request addresses all these instances.

[TimothyGu]

On another note, cmark handles this correctly.

[jgm]

Many thanks for the careful work!