Add Synonim Arrayflatten <-> Flatten #5764 (hcz)

Why it matters

This feature provides an alternative, shorter alias flatten for the arrayFlatten function, improving usability and readability of queries that require flattening nested arrays.

How to use it

Users can now use either arrayFlatten or flatten interchangeably in their SQL queries to flatten nested arrays. For example:

SELECT flatten([[1, 2], [3, 4]])

works the same as:

SELECT arrayFlatten([[1, 2], [3, 4]])