Add array_first and array_last functions

[gertjanal]

Description

Backstory: reason why element_at exists:
SELECT ARRAY['foo', 'bar'][10] throws array out of bounds exception
SELECT element_at(ARRAY['foo', 'bar'], 10) returns NULL, which is convenient

In many queries I use, I need the first element. However, as functions are nested, the arguments are not always very readable:
element_at(element_at(filter(persons, person -> person.age > 18), 1).addresses, 1).street

This PR introduces element_first and element_last:
element_first(element_first(filter(persons, person -> person.age > 18)).addresses).street

The element_last function can be used to return the last element.

Additional context and related issues

In Java, a List has getFirst and getLast as well: https://docs.oracle.com/en/java/javase/25/docs/api//java.base/java/util/List.html#getFirst()

Naming considerations

I considered first() and array_first(), but I think element_first() is better because it is much closer to the other function that people will use, the element_at() (also better when the client auto-suggests function names when typing element_)

Release notes

## General
* Add `array_first` and `array_last` functions

[raunaqmorarka]

lgtm
@martint PTAL

[martint]

Seems reasonable to add these. Let's rename the functions to array_first and array_last, though.

[Praveen2112]

Can we add additional test with nested array as well ?

[gertjanal]

I added

assertThat(assertions.function("array_first", "ARRAY_FIRST(ARRAY[ARRAY[1, 2], ARRAY[3, 4]]) "))
    .isEqualTo(1);

I think that is what you mean

[martint]

A couple of minor comments. Otherwise, it looks good. Once you apply the changes, I'll merge it. Thanks!

[martint]

The convention for function names is to use lower case: array_first

[martint]

same as above