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)

vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});