textsplitter: add an optional lenFunc to MarkdownTextSplitter

[pedramr]

Based on the custom lenFuc already supported in RecursiveCharacter splitter in 8734b6.

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[pedramr]

Thanks @FluffyKebab! Out of curiosity, what's the release cadence of the project? I see the latest release (v0.1.12) was cut on June 20.