Allow chunking on double-graphemes

### Description & Motivation

We would like to allow chunking on double graphemes, such as ?!

### Pitch

Currently, the chunk_text algorithm in textsplit.py assumes that we will be splitting on a single grapheme (! or. for instance). We would like to remove the assumption that this is the case.

### Alternatives

_No response_

### Additional context

To do this, `textsplit.py` and `text_config.py` would need to be refactored to accept strong and weak boundaries as lists. They are currently stored as strings.