Pin Haskell tool versions for VS Code

This also has the flake get its version pins (when possible) from the VS
Code settings. And we pin Cabal now, too.

.vscode/settings.json

@@ -0,0 +1,7 @@
+{
+  "haskell.toolchain": {
+    "cabal": "3.10.3.0",
+    "hls": "2.8.0.0",
+    "stack": "2.15.7"
+  }
+}

flake.nix

@@ -26,11 +26,15 @@
       "aarch64-darwin"
     ]
     (system: let
-      versions = {
-        ormolu = "0.7.2.0";
-        hls = "2.8.0.0";
-        stack = "2.15.7";
+      ## It’s much easier to read from a JSON file than to have JSON import from some other file, so we extract some
+      ## configuration from the VS Code settings to avoid duplication.
+      vscodeSettings = nixpkgs-release.lib.importJSON ./.vscode/settings.json;
+      versions =
+        vscodeSettings."haskell.toolchain"
+        ## There are some things we want to pin that the VS Code Haskell extension doesn’t let us control.
+        // {
         hpack = "0.35.2";
+        ormolu = "0.7.2.0";
       };
       pkgs = import nixpkgs-haskellNix {
         inherit system;

nix/haskell-nix-flake.nix

@@ -28,7 +28,7 @@
       tools =
         (args.tools or {})
         // {
-          cabal = {};
+          cabal = {version = versions.cabal;};
           ormolu = {version = versions.ormolu;};
           haskell-language-server = {
             version = versions.hls;