Add devcontainer configuration for development

Add a root-level devcontainer for developing the SQLite driver and
running all test suites. This is separate from the existing WordPress-
specific devcontainer in wordpress/.devcontainer/.

The configuration provides:
- PHP 8.4 with pdo_sqlite extension
- Composer (included in the base image)
- Node.js 20 (for Playwright E2E tests)
- Docker-in-Docker (for WordPress integration tests)

Author: JanJakes

.devcontainer/devcontainer.json

@@ -0,0 +1,13 @@
+{
+    "name": "SQLite Database Integration",
+    "image": "mcr.microsoft.com/devcontainers/php:8.4",
+    "features": {
+        "ghcr.io/devcontainers/features/node:1": {
+            "version": "20"
+        },
+        "ghcr.io/devcontainers/features/docker-in-docker:2": {
+            "moby": false
+        }
+    },
+    "postCreateCommand": "composer install"
+}