add nix flake

[honnip]

This pr introduces the nix packaging and simple dev shell.

This also allows nix run github:henriklovhaug/md-tui to run the package, which can be tested with nix run github:honnip/md-tui now.

[henriklovhaug]

Never used nix before, but I'll try it out. Should the lock file also be committed?

[honnip]

Yes, it's for reproducibility, but without it, it will display an annoying prompt I remember

[henriklovhaug]

Alright. Update changelog file, optionally include you name there, and I'll merge it in.

[henriklovhaug]

It should also be mentioned in the README

[honnip]

I've simply added a mention of flake because the way package is installed from flake depends on each user's environment.

I committed changes on github web editor, and I couldn't find how to overwrite the history. Squash merge please.