✨ implement SPLIT and SPLIT_STRING

[joocer]

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https://techcult.com/4-ways-to-split-string-by-delimiter-in-sql/

Split returns an array, split string returns a relation.

Split string should be a function relation, so can be used in from and joins, split just a standard function.

Both probably have the same implementation under the covers. https://arrow.apache.org/docs/python/generated/pyarrow.compute.split_pattern.html#pyarrow.compute.split_pattern

• [x] SPLIT
• [ ] SPLIT_STRING