Add support for the GAP language

Adds support for the GAP language, see http://www.gap-system.org/
Some repositories using this language:
- https://github.com/fingolfin/NormalizInterface
- https://github.com/gap-system/anupq
- https://github.com/neunhoef/orb
- https://github.com/homalg-project/AutoDoc

and in fact pretty much all repositories of these orgs / users:
- https://github.com/gap-system
- https://github.com/neunhoef
- https://github.com/homalg-project

I also made a pull request with Pygments for a GAP lexer, see https://bitbucket.org/birkenfeld/pygments-main/pull-request/311