Add a Nix template

[srid]

I changed my mind about https://github.com/srid/haskell-template/pull/98

But we should do it a limited manner. In particular nix flake init should only bring the following files:

• flake.nix
• fourmolu.yaml
• hie.yaml
• justfile
• Jenkinsfile
• .hlint.yaml
• .envrc

Basically help automate modernization work like https://github.com/srid/commonmark-simple/pull/1/commits/4339af157a16e24d68238a39d35da996783be4f9

[srid]

You can now run:

nix flake init -t github:srid/haskell-template

This includes all files. A better way to initialize the template will be provided as part of https://github.com/juspay/omnix/issues/152

[srid]

A better way to initialize the template will be provided as part of juspay/omnix#152

https://omnix.page/om/init.html