Add support for .env shells to flake.nix (#3322)

The main motivation for this change is to enable development
of HLS plugins by generating a `.env` shell for every package in
this repository.

Example usage:

```ShellSession
$ nix develop .#haskell-language-server-dev-env.hls-fourmolu-plugin
```

This is a plain shell, though, without any tooling installed, identical
to the one that Nixpkgs provides by default.

Author: Gabriella439

flake.lock

@@ -362,6 +362,10 @@
 
             src = null;
           };
+
+        mkEnvShell = hpkgs:
+          pkgs.lib.mapAttrs (name: value: hpkgs.${name}.env) pkgs.hlsSources;
+
         # Create a hls executable
         # Copied from https://github.com/NixOS/nixpkgs/blob/210784b7c8f3d926b7db73bdad085f4dc5d79428/pkgs/development/tools/haskell/haskell-language-server/withWrapper.nix#L16
         mkExe = hpkgs:
@@ -394,14 +398,23 @@
           haskell-language-server-942-dev-nix = mkDevShellWithNixDeps ghc942 "cabal.project";
         };
 
+        # The default shell provided by Nixpkgs for a Haskell package (i.e. the
+        # one that comes in the `.env` attribute)
+        envShells = {
+          haskell-language-server-dev-env = mkEnvShell ghcDefault;
+          haskell-language-server-902-dev-env = mkEnvShell ghc902;
+          haskell-language-server-924-dev-env = mkEnvShell ghc924;
+          haskell-language-server-942-dev-env = mkEnvShell ghc942;
+        };
+
         allPackages = {
           haskell-language-server = mkExe ghcDefault;
           haskell-language-server-902 = mkExe ghc902;
           haskell-language-server-924 = mkExe ghc924;
           haskell-language-server-942 = mkExe ghc942;
         };
 
-        devShells = simpleDevShells // nixDevShells // {
+        devShells = simpleDevShells // nixDevShells // envShells // {
           default = simpleDevShells.haskell-language-server-dev;
         };