Add Vitest configuration for testing and coverage

vitest.config.js

@@ -0,0 +1,12 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html', 'lcov'],
+      include: ['trmnl-form-builder.js']
+    }
+  }
+})