chore: add the lean4 extension to the vscode workspace (#3059)

This prompts users opening the workspace (on a new device) for the first
time to install the lean extension

# Summary

Link to `RFC` or `bug` issue: N/A

lean.code-workspace

@@ -48,5 +48,10 @@
 				}
 			}
 		]
+	},
+	"extensions": {
+		"recommendations": [
+			"leanprover.lean4"
+		]
 	}
 }