Add recommended list of extensions for vscode

Author: smelc

.vscode/extensions.json

@@ -0,0 +1,8 @@
+{
+  "recommendations": [
+    "formulahendry.code-runner",
+    "mhutchie.git-graph",
+    "haskell.haskell",
+    "justusadam.language-haskell"
+  ]
+}