chore: Don't allow Title in element

[Muscraft]

Having Title as an Element did not make sense, as Title is only meant to be used with Group::with_title, and Message should be used if you want similar output. Given that within our own tests, we were using Title when we should've been using Message, having both would have no doubt led to confusion for users over what to use.

Note: this adds a bit of duplication for how we handle continuations; the next few PRs should hopefully address some of it.

[epage]

How come you didn't do the public/private enums?

[Muscraft]

Having it be separate keeps the internal implementation in sync with the API, making it easier to reason about. I also found it nice to have everything for Title self-contained, as you don't have to think about it in relation to the other Elements.