Pygments lexer for HOL

[digama0]

Now that we have moved to zulip, the fact that HOL lacks a syntax highlighter is more pressing, since the pygments SML parser is less forgiving than the one used by discord. We should submit a lexer to pygments.

• Example lexer: https://github.com/pygments/pygments/blob/master/pygments/lexers/ml.py
• Contribution instructions: https://pygments.org/docs/lexerdevelopment/