feat: add onInit prop with editor instance

[tjwelde]

fixes #163
fixes #144

This PR adds an onInit hook, which exposes the underlying tiptap editor instance to the parent component, needed for customizations.

As alternative I was thinking, that it would be nice to expose a useEditor hook, similar to how tiptap does it, just with the novel enhancements already included. This could be added additionally to the onInit hook in this PR (so no matter the decision, we could just merge this.)
#136 seems to go into a similar direction, but does much more. If you are interested in a lean hook alternative, I could make a PR for that, too.

[vercel]

@tjwelde is attempting to deploy a commit to the Elegance Team on Vercel.

A member of the Team first needs to authorize it.

[josealvarez97]

@tjwelde Did you find a way to access the editor instance in parent components?

C.C. @steven-tey: Awesome work Steven, this is so good! 🙂

I think this feature would be particularly useful to continue leveraging the full capacity of TipTap on top of Novel.

There seem to be other people asking for the same feature:

• Expose Internal Editor State for External Use #163
• how to get Editor ref #196

Alternatively, is there a way to access the editor instance that I am missing?

[andrewdoro]

https://novel.sh/docs/components/editor-content onCreate callback for getting the editor instance