Improve highlighting for aggregate functions

[jorisroovers]

Like SUM, AVG, etc.

While these tend to be database specific, there are also a bunch that are almost universally supported.

Example reference from DuckDB: https://duckdb.org/docs/sql/aggregates

Similar comment wrt highlighting other functions, eg: https://duckdb.org/docs/sql/functions/overview

Haven't looked at the source so not sure if it's easy to do different highlighting depending on different SQL dialects (perhaps this can be discovered and/or configurable?).

Regardless of dialect, I think it's pretty safe to always highlight the most commonly supported aggregate functions (like SUM, AVG, COUNT, etc). Perhaps in a different color than other SQL keywords?