Use v8 coverage

vite.config.ts

@@ -17,7 +17,7 @@ export default defineConfig({
     // TODO: should we move this to a mermaid-core package?
     setupFiles: ['packages/mermaid/src/tests/setup.ts'],
     coverage: {
-      provider: 'istanbul',
+      provider: 'v8',
       reporter: ['text', 'json', 'html', 'lcov'],
       reportsDirectory: './coverage/vitest',
       exclude: ['**/node_modules/**', '**/tests/**', '**/__mocks__/**'],