rfctr: rename pre chunk (#2261)
The original naming for the pre-cursor to a chunk in `chunk_by_title()`
was conflated with the idea of how these element subsequences were
bounded (by document-section) for that strategy. I mistakenly picked
that up as a universal concept but in fact no notion of section arises
in the `by_character` or other chunking strategies.
Fix this misconception by using the name `pre-chunk` for this concept
throughout.
