test: add Vitest + 30 dice-engine tests

- Install vitest and @vitest/coverage-v8
- Add test scripts to package.json
- Create vitest.config.ts
- Add 30 comprehensive tests for parseDiceNotation, rollDice,
  rollAdvantage, buildNotation, isAdvancedNotation
- Fix isAdvancedNotation regex to detect ro (reroll once)

package.json

@@ -5,12 +5,19 @@
   "scripts": {
     "dev": "astro dev",
     "build": "astro build",
-    "preview": "astro preview"
+    "preview": "astro preview",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "@astrojs/sitemap": "^3.2.1",
     "@tailwindcss/vite": "^4.0.0",
     "astro": "^4.16.0",
     "tailwindcss": "^4.0.0"
+  },
+  "devDependencies": {
+    "@vitest/coverage-v8": "^4.1.5",
+    "vitest": "^4.1.5"
   }
 }