Add configuration for Nix Environment Selector

This is a VS Code extension that I use to make rust-analyzer work with
the `shell.nix` that I added in the previous commit.

.vscode/settings.json

@@ -7,5 +7,6 @@
     "rust-analyzer.diagnostics.disabled": [
         "mismatched-arg-count"
     ],
-    "rust-analyzer.check.command": "clippy"
+    "rust-analyzer.check.command": "clippy",
+    "nixEnvSelector.nixFile": "${workspaceRoot}/shell.nix"
 }