rfctr(chunking): extract BasePreChunker (#2294)
The `_split_elements_by_title_and_table()` function fulfills the
pre-chunker role for `chunk_by_title()`, but most of its operation is
not strategy-specific and can be reused by other chunking strategies.
Extract `BasePreChunker` and use it as the base class for
`_ByTitlePreChunker` which now only needs to provide the boundary
predicates specific to that strategy.
