Chunk overlap prefix is on even word boundary >= overlap character count.

**Problem**
Chunk text begins mid-word when `overlap` is specified.

![image](https://github.com/Unstructured-IO/unstructured/assets/39398937/cc2393e3-ce78-4b3f-a541-b6c9d6854481)

**Desired solution**
Compute the overlap prefix as the next even-word boundary greater than or equal to `overlap` characters from the end of the prior chunk.