additional string `split` functions

[universalmind303]

Description

Add a few additional functions for splitting strings into arrays.

create table strings (v text);
insert into strings values ("foo bar"), ("foo_bar") ("foo bar baz");

Note: this is the current string_to_array function, but i think split is much more intuitive.

• split: Split the string by a substring.

select split(v, ' ') from values;
----
["foo", "bar"]
["foo_bar"]
["foo", "bar", "baz"]

• splitn: Split the string by a substring, restricted to returning at most n items.

select splitn(v, ' ', 2) from values;
----
["foo", "bar"]
["foo_bar"]
["foo", "bar baz"]