Recursive UNNEST for Nested Data

When lists contain other lists, a single `UNNEST` only peels off one layer. Use the `recursive` parameter to flatten all levels at once.

SELECT unnest([[1, 2], [3, 4, 5]], recursive := true)

Without `recursive` this returns two rows — each inner list as a value. With `recursive := true` it returns five rows — the individual integers.

This works on structs too. A list of structs with recursive unnesting first expands the list into rows, then expands each struct into columns:

SELECT unnest([{'a': 1, 'b': 2}, {'a': 3, 'b': 4}], recursive := true)

Note that lists inside structs are not unnested — recursive unnesting first fully unnests all lists, then fully unnests all structs.