add support for unicode angle brackets, single vertical bar, double vertical bar

[kevinbarabash]

This PR also allows these symbols to be used with left/right delimiters. This should allow us to finally close #243.

TODO: write some tests

[ylemkimon]

Just out of curiosity, is it valid to have right delimiters, e.g., \rangle, as a left delimiter?

[kevinbarabash]

I checked with quicklatex.com and it appears so although I'm not sure in what context you'd actually want to do that.

[ronkok]

PR #993 and unicode-math both map U+2223 to \mid, not to \lVert or \rVert. \mid is an operator, not a delimiter, useful for set builder notation.

The single character analogue to \vert is ASCII "|", U+007C. That's been true since the TeXbook, p146.

[kevinbarabash]

PR #993 and unicode-math both map U+2223 to \mid, not to \lVert or \rVert. \mid is an operator, not a delimiter, useful for set builder notation.

@ronkok thanks for catching this. I'll remove the \lvert/\rvert changes from this diff.

[ronkok]

Similarly, unicode-math maps U+2225 to \parallel, which is also an operator, not a delimiter.

[kevinbarabash]

I think this could be extend to something like:

const cases = {
    'shouldParse': [ ... ],
    'shouldBuild': [ ... ],
    'shouldNotParse': [ ... ],
    'shouldNotBuild': [ ... ],
    'shouldParseLike': [ ... ],
    'shouldBuildLike': [ ... ],
};

I'll open a separate PR for this.