Add lexer for Promela

[lmarzen]

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

I have opened a similar pull request for Pygments: pygments/pygments#2620
I generated the lexer using my branch of Pygments with Promela.

$ python3 -m venv pygments-promela
$ source pygments-promela/bin/activate
$ pip install pystache
$ pip install /path/to/pygments-promela 

$ python3 _tools/pygments2chroma_xml.py pygments.lexers.c_like.PromelaLexer > lexers/embedded/promela.xml

Verified expected formatting using go run . Playground.

[alecthomas]

Would you mind adding some test text please?

[alecthomas]

Thanks!

[lmarzen]

Thank you, great project you have going here!