Add parser support for STRING_AGG()

[lukaseder]

We're currently not parsing STRING_AGG yet, e.g.

SELECT string_agg(a, ',')
FROM (VALUES (1), (2)) AS t(a)

Produces:

Unknown function: [1:19] SELECT string_agg([*]a, ',')
FROM (VALUES (1), (2)) AS t(a)

It's a SQL Server function with optional WITHIN GROUP clause:
https://learn.microsoft.com/en-us/sql/t-sql/functions/string-agg-transact-sql?view=sql-server-ver16