feat: statistical chunker improvements

[simjak]

Improved async method for statistical chunker
call duration: 46.23 seconds
acall duration: 2.05 seconds

However, I noticed some minor differences in how the text is split between sync and async. See the notebook in the docs

[github-actions]

Failed to generate code suggestions for PR

[jamescalam]

@simjak this is great, could I just ask we do a couple minor things before merging:

• keep the original chunking notebook (ie sync) as notebook 00 but then add the async notebook as a new example — we can add references/links to 00 along the lines of "Note: by using the [async methods here](link) docs can be processed *40x* faster." — this way we show examples for both sync and async while keep the notebooks simple and also demonstrating how much faster async can be.
• resolve lint issues :)

[jamescalam]

resolves #15