feat!: strict equality operator

[CertainLach]

Just remembered I have implemented a test version of that a long time ago, it is very outdated, and I haven't finished some of the changes I wanted, such as wrong formatting due to sqlformat crate at the time not supporting IS NOT DISTINCT FROM operator:

from users
filter a === b

SELECT
  *
FROM
  users
WHERE
  a IS NOT DISTINCT
FROM
  b

Will rebase it to the master soon

Fixes: https://github.com/PRQL/prql/issues/4289