Add custom sql functions

[elcritch]

Adds basic support for custom sql functions using importSql pragma:

proc substr(s: string; start, length: int): string {.importSql.}

proc exampleQuery() =
  let prefix = "Mr"
  let res = query:
    select tb_string(substr(typstring, 0, 2))
    where substr(typstring, 0, 2) == ?prefix
  let expected = ss.mapIt($(toRunes(it)[0..1]))
  check res == expected

[elcritch]

Be nice to upgrade these at some point to also support type checking the args. But alas, not today.

[elcritch]

I used GPT5-Codex-High for this. Seems to have done a good job. It checked for conditions I wouldn't have thought to check. I had to cleanup a little bit in the tests.