Create devcontainer.json

I've been using Github codespaces for development (bc I have a very old macbook air). Figured it may be nice to include a config for it in the repo so that forkers can easily spin up a dev environment that closely matches that of the code maintainers.

.devcontainer/devcontainer.json

+{
+  "image": "mcr.microsoft.com/devcontainers/universal:2",
+  "features": {
+    "ghcr.io/devcontainers/features/node:1": {},
+    "ghcr.io/devcontainers-contrib/features/turborepo-npm:1": {},
+    "ghcr.io/devcontainers-contrib/features/typescript:2": {}
+  }
+}