[Feature Request]: Add Missing PGSQL Functions

[elliothomerwilkinson]

Topic

Support new SQL

Description

The PostgreSqlLexer.g4 is missing support for common Postgres functions TO_CHAR and DATE_TRUNC

Please can these be added.

This is an issue since basic postgres sql like below is marked as invalid:

SELECT
    TO_CHAR(DATE_TRUNC('week', "my_date"), 'YYYY-MM-DD') as x
FROM test_table;