Update dev env

[SnO2WMaN]

While trying to work out a fix for the problem with https://github.com/dhall-lang/vscode-dhall-lsp-server/issues/37 (though this effort was mostly wasted, as the problem was probably on the lsp side), I found a couple of some outdated or deprecated aspects of the project itself, the development environment, etc., so I fixed them. The main things I did were

• Development environment with Nix (now, just hit direnv allow and most of the setup is done)
• Code formatting with dprint/treefmt
• Replacing tslint with ESLint