add vitest configuration file

packages/markdown-splitter/vitest.config.ts

@@ -0,0 +1,25 @@
+import tsconfigPaths from 'vite-tsconfig-paths';
+import { defineConfig, coverageConfigDefaults } from 'vitest/config';
+
+export default defineConfig({
+  plugins: [
+    tsconfigPaths(),
+  ],
+  test: {
+    environment: 'node',
+    clearMocks: true,
+    globals: true,
+    coverage: {
+      exclude: [
+        ...coverageConfigDefaults.exclude,
+        'src/**/index.ts',
+      ],
+      thresholds: {
+        statements: 100,
+        branches: 100,
+        lines: 100,
+        functions: 100,
+      },
+    },
+  },
+});