Add WebKit to e2e test matrix (#37298)

Verified locally with 50 runs, averaging 9 seconds per local test suite
run. Total suite took 15s.

`--with-deps` is needed because webkit's dependencies are not
pre-installed on GHA runners (as opposed to firefox/chrome which are
preinstalled).

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>

playwright.config.ts

@@ -36,5 +36,11 @@ export default defineConfig({
         ...devices['Desktop Firefox'],
       },
     },
+    {
+      name: 'webkit',
+      use: {
+        ...devices['Desktop Safari'],
+      },
+    },
   ],
 });