Rewrite validate.sh in Haskell

[9999years]

validate.sh is the entry-point to CI and local testing. validate.sh is needed because it's non-trivial to run tests otherwise.

However, validate.sh is a shell script, which limits its expressive power and maintainability.
I've been working on improving it with PRs like #10293 and #10315, but working with shell scripting idioms makes it much more difficult than it needs to be. Let's rewrite it as a simple Haskell script. I'm willing to do this if the maintainers OK it.

[philderbeast]

Are you considering using something like turtle?

[9999years]

@philderbeast Yes, but I'm not set on it if the maintainers have other preferences.

[philderbeast]

When generating a sha256map for nix, turtle came in handy and was easy to use so I'd be happy to see it used here. Another thing I did with Updo was offer the option to install each standalone *.hs script by listing them as executables in the package. That could help speed thing up for scripts called multiple times.

[ulysses4ever]

I'm in support. But we should try to think really hard of potential adversarial effects. Portability. Startup times. Downstream users including distributors like ghcup and nixpkgs (who may want to run it). Nothing jumps at me as a show stopper.

At least, probably, validate.sh should stay and turn into a one-liner calling validate.hs?