Auto merge of #26710 - AbhishekSharma102:issue_26571, r=jdm,gterzian

Compile external scripts off the main thread

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #26571  (GitHub issue number if applicable)

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resources/prefs.json

@@ -20,6 +20,7 @@
   "dom.offscreen_canvas.enabled": false,
   "dom.permissions.enabled": false,
   "dom.permissions.testing.allowed_in_nonsecure_contexts": false,
+  "dom.script.asynch": true,
   "dom.serviceworker.enabled": false,
   "dom.serviceworker.timeout_seconds": 60,
   "dom.servo_helpers.enabled": false,