Implement 0.39.0's regex replace

[PgBiel]

Sample impl:

let
   reRepl = from: to: str: builtins.concatStringsSep "" (builtins.map (x: if builtins.isList x then to else x) (builtins.split from str))
in reRepl "a+" "b" "Hello aaaaaaaaaaa the aaaaaaaa the a"