Add Haskell extension to devcontainer config

Author: Porges

.devcontainer/devcontainer.json

@@ -12,7 +12,9 @@
 	"settings": {},
 
 	// Add the IDs of extensions you want installed when the container is created.
-	"extensions": [],
+	"extensions": [
+		"haskell.haskell"
+	],
 
 	// Use 'forwardPorts' to make a list of ports inside the container available locally.
 	// "forwardPorts": [],