Add REGEXP_REPLACE support (including unit test)

[Zodiac1978]

Fixes #47

[Zodiac1978]

Happy to get your review @bgrgicak ! :)

[Zodiac1978]

@adamziel I have now additionally added checks for if any of the required parameters are null.

Copying the result from MySQL:

If expr, pat, or repl is NULL, the return value is NULL. https://dev.mysql.com/doc/refman/8.4/en/regexp.html#function_regexp-replace

[adamziel]

Actually, let’s also test for empty string pattern, not just a null one

[adamziel]

I just had a moment to review this in a more focused setting and I left a few more notes. It is pretty close!