Keyword completion: Inquire `pg_catalog.pg_proc` table for scalar functions

## About

CrateDB offers a PostgreSQL-compatible `pg_catalog.pg_proc` database system table. It could be used to inquire all scalar function symbols, to add them to the list of keywords provided by the `crate.crash.repl.SQLCompleter`, similar to how the `pg_catalog.pg_get_keywords()` function is used to inquire the list of non-function keywords.

## Outlook

At GH-360, we also reported:

> Some scalar functions like `floor` or `ceil` are included within the documentation, but are not part of either `pg_catalog.pg_get_keywords()` nor the list of fallback keywords [4].
> [4] https://github.com/crate/crash/blob/0.27.0/crate/crash/repl.py#L115-L151

In this spirit, other than using `pg_catalog.pg_proc`, the list of "fallback keywords", or maybe a list of "additional keywords", would probably still need to be maintained, in order to accompany for any gaps.