chore: add devcontainer with nix feature for dev shell

Author: iamralch

.devcontainer/devcontainer.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://raw.githubusercontent.com/devcontainers/spec/refs/heads/main/schemas/devContainer.base.schema.json",
+  "name": "sqlc-gen-queries",
+  "image": "mcr.microsoft.com/devcontainers/base:bookworm",
+  "workspaceFolder": "/workspaces/sqlc-gen-queries",
+  "workspaceMount": "source=${localWorkspaceFolder},target=/workspaces/sqlc-gen-queries,type=bind",
+  "features": {
+    "ghcr.io/devcontainers/features/docker-in-docker:2": {},
+    "ghcr.io/devcontainers/features/nix:1": {
+      "extraNixConfig": "experimental-features = nix-command flakes"
+    }
+  }
+}