add devcontainer.json for codespace (#32)

Author: clstokes

.devcontainer/with-tailscale/devcontainer.json

@@ -0,0 +1,7 @@
+{
+  "name": "Codespace with Tailscale",
+  "runArgs": ["--device=/dev/net/tun"],
+  "features": {
+    "ghcr.io/tailscale/codespace/tailscale": {}
+  }
+}