feat: add array_first higher-order array function

Add `array_first(array, predicate)`, a higher-order function that returns
the first element of an array for which the lambda predicate returns true.
It returns null when the array is empty or no element matches; a predicate
that returns null for an element is treated as not matching, and a matched
null element is returned as null.

Implemented as a `HigherOrderUDFImpl` alongside the existing array lambda
functions (`array_any_match`, `array_filter`, `array_transform`), with
`list_first` as an alias. Includes unit tests, sqllogictest coverage, and
generated documentation.

Author: EdsonPetry