Add support for checking if a syntax is registered

[wooorm]

1. why is it needed?
2. could you add docs (edit: done!) and tests?
3. Prism uses different terms compared to highlight.js (e.g., register instead of registerLanguage), so we could use language or syntax or something else here too, what do you think?

[conorhastings]

@wooorm added tests

in regards to your first question, I'd like a way to know if syntax has been previously registered or not.

[conorhastings]

@wooorm updated with tests and coverage passing

[conorhastings]

@wooorm fixed example

[wooorm]

This looks great!

One more Q, while I was in the process of merging this: the result of syntax isn’t useful at all to other people I think. How about a registered function instead, that returns a boolean: whether the given language is registered?

Does that solve your problem?

[conorhastings]

@wooorm yeah would actually prefer that, just was matching lowlight as much as possible, but i'll update to that

[wooorm]

Don’t worry, I’ll do it, I was half way through merging this already!

[conorhastings]

@wooorm sounds good, thanks!

[wooorm]

Released!

[conorhastings]

@wooorm awesome! thanks!