test: remove webkit & firefox

- firefox has a tendency to hang, which makes testing difficult
- safari has some support differences which make initial test creation a bit less convenient

I'd like to re-add both these once vitest's browser mode is a bit more stable & most of restyle's tests are passing

vitest.config.ts

@@ -16,11 +16,7 @@ export default defineConfig({
       provider: 'playwright',
       headless: true,
       screenshotFailures: false,
-      instances: [
-        { browser: 'chromium' },
-        { browser: 'firefox' },
-        { browser: 'webkit' },
-      ],
+      instances: [{ browser: 'chromium' }],
     },
   },
 })