Add support for STRING_AGG <-> GROUP_CONCAT transpilation

[georgesittas]

Fixes #754

This seems to do the trick, although I'll take a look an try to simplify/refactor things in a bit.

[georgesittas]

@tobymao
Ok, I'd say this is good to go. Not sure if there's any part that can be greatly simplified. Seems like STRING_AGG is a T-SQL function so I think it makes sense to not move these implementations over to the base parser / generator. Scratch that, it's also supported in postgres, so I moved the parse string agg method to the base parser.

Let me know if I've missed anything.

[georgesittas]

Sources for sqlite/postgres additions:

• https://www.postgresql.org/docs/9.0/functions-aggregate.html
• https://www.sqlite.org/lang_aggfunc.html#group_concat

We may be able to dry up the generator method for group concat / string agg.

Edit: regarding the postgres NULLS FIRST/LAST form, I checked locally for postgres15 and it seems to work too (although I'm not 100% sure about the exact semantics), so I didn't try to change it in order to match the other generator methods.