Use `cluffer:buffer`s in `input-edit-mode`

[aartaka]

Create full cluffer:buffers, instead of cluffer:lines. This way we will make it more performant and simple (I've just tried to implement cursor-next-line and was overwhelmed by the complexity of that). After all, cluffer distinguishes between buffer and line for a reason.

Current implementation is quite slow on long text fields, too.

[Ambrevar]

Makes sense to me. @jmercouris ?

[jmercouris]

Yes, makes sense to me too!

[aadcg]

Yes, this is indeed a mistake that probably derived from not carefully studying the cluffer library.

Given that we have zillions of ways to have editor-like behaviors in Nyxt (each with its own set of quirks), we should probably firstly understand what we want.