Add lexer for Promela

[lmarzen]

This pull request adds a lexer for Promela. (https://spinroot.com/spin/whatispin.html)

Promela (Process Meta Language) is a C-like language used by SPIN to formally model software and hardware systems. SPIN (Simple Promela Interpreter) was originally developed in the 1980s at Bell Labs and since 1991 has been open-source and is a popular formal methods tool.

Pygments already supports other popular formal methods languages, including Isabelle, Coq, and NuSmv. Promela would be a fitting addition.

I have not included test snippets since PromelaLexer is derived from CLexer, though I have tested that it produces correct highlighting on a few of my own Promela files. Edit: I added snippets and examplefiles and ran tox -e check and tox -e mapfiles which both succeeded.

[cmarqu]

(Another potential pygments contributor here...)
You might want to add a line to the CHANGES file also.

[jeanas]

You might want to add a line to the CHANGES file also.

Well, we (maintainers) update it ourselves; we don't ask contributors to do this. (But thank you for taking a look at the PR.)

[jeanas]

Looks OK. Since it seems this language can contain embedded C, shouldn't 'types' and 'keywords' have an inherit at the end to also highlight C types/keywords?

[lmarzen]

Promela does not have all the types and keywords of C. Only the ones I specified, so inherit, was initentially left out.

[jeanas]

Sorry for the delay. Thanks for your contribution!