Add an "Advanced" flake template

[silky]

Fixes #18

This is a draft @cdepillabout because I'm not sure if it's a great idea; interested in feedback.

Pros:

• Well, it's a flake template, and you can run nix flake init --template ~/dev/stacklock2nix#advanced to install it.
• It can be built with any of the "expected" mechanisms.

Cons:

• In order to make it build, I had to copy in an entire stack project skeleton.
• It's pinned to a specific LTS,
• A dummy some-app name is a bit polluted all over the place.
• It duplicates a bit code that's already in the repo; in terms of say this example (maybe we should just turn that into the template?)
• I do an opinionated thing with cabal, (make it call hpack first) that people might not like.

Maybe, upon reflection, a stack template (from a URL) would be more stack-idiomatic, as at least then some of those variables can be set around in various places in the nix files.

TODO:

• Decide if this is a good idea or not
• Resolve todo notes in code

Open to opinions ...

[silky]

@cdepillabout Closing this just to not leave it stale; but let me know if you have any thoughts on how to move forward in a way that you would prefer!

[cdepillabout]

@silky Hey sorry, I've been pretty busy the last couple weeks. I do have reviewing this PR on my TODO list, but I may not get to it for another two weeks or so.

Feel free to leave it open though. Or if you'd rather close it, I can reopen it when I get to it.

[silky]

Closing; I think it's fair to say this isn't the way to go.