API for supported languages/plugins/etc.

[RunDevelopment]

Motivation
Libraries sometimes need to know all languages/plugins/etc. Prism supports.

Description
A simple API to:

1. Check whether a given language/plugin/etc is supported.
2. Query all languages/plugins/etc.

My suggestion is to make this a part of components.js. Something like:

components.supported(lang: string | string[]): boolean;
components.supported.languages: string[];
components.supported.plugins: string[];

The other properties of the components object are still private to Prism. By implementing the API in components.js, both browser and NodeJS devs have easy access to it.

Alternatives
Document components.json and declare it to be a part of Prism's public API. Right now, it's considered a private part of Prism, so we could make it public.
This has the obvious downside that changes to components.json might be breaking and it takes a lot of freedom from Prism's core development team.