Add regexp_replace function

[Alexnortung]

Here's the implementation that worked for me. I am unsure if there are some other things to be aware of between postgres and js when doing regex.

I just think it would be nice to have it out of the box from pg-mem :)

    db.public.registerFunction({
        name: 'regexp_replace',
        args: [DataType.text, DataType.text, DataType.text, DataType.text],
        returns: DataType.text,
        implementation: (x: string, pattern: string, replacement: string, flags: string) => x?.replace(new RegExp(pattern, flags), replacement),
    });