Add ANTLR language

[BillWagner]

Fixes #1290

I did not implement all the styles in that issue, but this does correctly recognize ANTLR, highlight grammar rules, recognize comments, and add ANTLR as a first class citizen for the set of languages that can be processed.

[KvanTTT]

ANTLR has much more keywords, you changes include only 4 ones. Moreover, you don't distinguish between TOKEN and parse tree rules.

Please take a look on my highlighting scheme Antlr4.xshd.

[joshgoebel]

@BillWagner

First of all, truly sorry for the long delay in anyone responding to this PR.

Thank you so much for desiring to contribute and writing a language grammar. Unfortunately, we currently are no longer accepting new languages into the core repository. The existing library maintainers don't simply have time to maintain any new languages (fixing bugs, dealing with issues, etc). We'd still love to host your language as a 3rd party module though - if you're willing to help maintain it.

It's easy to turn your language into a module that anyone who likes can easily integrate with Highlight.js using a single line of code:

<script src="/path/to/highlight.min.js"></script>
<!-- they'd simply add this single line -->
<script src="/path/to/highlightjs-antlr.js"></script> 
<script>hljs.initHighlightingOnLoad();</script>

If you'd like to become the maintainer for this language then we're happy to create a new repository in the highlightjs organization and host the repository there. That assumes you'd be open to reviewing issues and fixing bugs as they arise. You're also welcome to host the repo yourself and we'll link to it from our README.

There isn't much in the way of how to docs on this yet, but there are sample repositories and it's honestly pretty easy. We're still fleshing out an official "spec" for 3rd party modules, but even after that's done it should require minimal changes from the examples we have already. IE, if you wanted to get started now there isn't any big reason to wait.

My own robots.txt grammar repository is a great, simple example:

https://github.com/highlightjs/highlightjs-robots-txt

If you have any questions feel free to ask.

[joshgoebel]

Closing for inactivity; (also finally cleaning all these old PRs out for the new year).

As mentioned above if you're interested in becoming the maintainer of a 3rd party module, please respond [whenever you find time] and we'd be happy to help you get started.