feat: implement Phase 1 - Core Page Cache and Admin Settings UI

Implement the core page caching functionality and admin settings interface
for WP Recache plugin. This includes:

- Plugin bootstrap with activation/deactivation handlers
- Admin settings page with tabbed interface (Cache, Optimization, Advanced)
- Admin bar integration for quick cache purging
- Core page cache using output buffering
- Cache file management with mobile device separation
- Cache purge on post update/comment
- Default URL exclusions (cart, checkout, account)
- Cache statistics display
- PHPUnit test suite setup
- Docker-based development environment
- PHPStan configuration for static analysis

References:
- Closes #1 (Core Page Cache)
- Closes #8 (Admin Settings UI)

composer.json

@@ -0,0 +1,45 @@
+{
+    "name": "kevin-bataille/wp-recache",
+    "description": "Open source WordPress performance plugin - page caching, file optimization, image optimization",
+    "type": "wordpress-plugin",
+    "license": "GPL-2.0-or-later",
+    "authors": [
+        {
+            "name": "Kevin Bataille",
+            "email": "noreply@gmail.com"
+        }
+    ],
+    "require": {
+        "php": ">=7.4",
+        "ext-dom": "*",
+        "ext-libxml": "*"
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^9.5",
+        "wp-phpunit/wp-phpunit": "^6.0",
+        "yoast/phpunit-polyfills": "^1.0",
+        "phpstan/phpstan": "^1.10"
+    },
+    "autoload": {
+        "classmap": [
+            "includes/"
+        ]
+    },
+    "autoload-dev": {
+        "classmap": [
+            "tests/"
+        ]
+    },
+    "scripts": {
+        "test": "phpunit",
+        "test:coverage": "phpunit --coverage-html coverage",
+        "phpstan": "phpstan analyse",
+        "lint": "phpcs --standard=WordPress"
+    },
+    "config": {
+        "allow-plugins": {
+            "dealerdirect/phpcodesniffer-composer-installer": true,
+            "phpunit/phpunit": true
+        }
+    }
+}