Add support for language NATURAL

[MarkusAmshove]

This PR adds support for the language NATURAL by Software AG.

Sadly, it is a proprietary language, but you can see some example code e.g. here

[MarkusAmshove]

should be detected as cpp:

      AssertionError: expected 'natural' to be 'cpp'      

      actual expected

      

      naturalcpp

I'm not sure what happened here.

[joshgoebel]

Sadly, due to a lack of time and maintainers we are generally not accepting new languages into the core library. Feel free to publish your library as a language extension though, and if you'd like to be this languages maintainer we can create even a repository here inside the highlightjs organization (for discoverability), if you would like. It's reasonably easy for someone using highlightJS to load additional languages simply by including a JS tag for them and then using hljs.registerLanguage().

There are no dedicated instructions but you can take a look at an example such as:
https://github.com/highlightjs/highlightjs-robots-txt

Related discussion:
#2149

Feel free to join the discussion.