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
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- [ ] These changes fix #26571  (GitHub issue number if applicable)

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components/config/prefs.rs

@@ -192,6 +192,9 @@ mod gen {
                         allowed_in_nonsecure_contexts: bool,
                     }
                 },
+                script: {
+                    asynch: bool,
+                },
                 serviceworker: {
                     enabled: bool,
                     timeout_seconds: i64,