Add a dev container definition

Author: nex3

.devcontainer/devcontainer.json

@@ -0,0 +1,6 @@
+{
+	"image": "mcr.microsoft.com/devcontainers/universal:2",
+	"features": {
+		"ghcr.io/devcontainers-community/templates/dart:1": {}
+	}
+}