improve max length handling

[jordanxlau]

Bug description

Currently, the algorithm in textsplit.py will cut a sentence off at the max_length, even if this splits up a word. Instead, this algorithm should always preserve words and perhaps even preserve whole sentences or phrases.

How to reproduce the bug

# Sample code to reproduce the problem

Error messages and logs

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Environment

Current environment

# Please paste the output of `everyvoice --diagnostic` here
# EveryVoice Diagnostic information

More info

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