[ML] Add code chunking strategy

[dan-rubinstein]

Description

Our existing chunking strategies are optimized around blocks of written text but are not optimized for handling code. The purpose of this issue is to investigate alternate chunking strategies we could offer for handling code blocks and implement such a strategy for our users.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[dan-rubinstein]

We should be able to reuse the recursive chunker being added as part of this issue for this.

[dan-rubinstein]

Closing this issue as we have implemented the recursive chunker which allows users to define their own settings for various structured languages (including coding languages). We can reopen this if we ever want to expand this for specific languages.