Discuss: Long-term discussions index

This issue was created to have a single place for these long-lived/winded topics to live rather than having multiple un-closable issues. Discussions end up here if there is no immediate "next step" but the discussion itself remains contextually valuable to have around.

- [Discuss: Plug-in hooks and building a plug-in culture](https://github.com/highlightjs/highlight.js/issues/2225)
- [Discuss: Auto-detection discussion and it's difficulties](https://github.com/highlightjs/highlight.js/issues/1213)
- [Policy: Discuss extras repository for additional languages](https://github.com/highlightjs/highlight.js/issues/2149) (why 3rd party grammars)

The overall discussion can continue in these threads and when/if more discrete accomplishable tasks/issues are determined new issues can be opened for those smaller tasks.

_This issue itself is not a discussion._