regex builtins

[fogti]

Hi, I've written https://github.com/zseri/nix-builtin-utils (when it hits a stable state I'd like to move it to the nix-community org, but it's not there yet...), which provides a C interface for the regex API which the original Nix implementation uses for builtin.match and builtin.split (look at the test suite for usage examples). These proved to be difficult when trying to transpile Nix to Javascript, and also when trying to implement it in Rust (otherwise I could've just compiled a rust impl to WASM, and get it done that way...) because most regex engines differ, so that they can cause subtle breakage...