Fix finalizer in ExternalStringImpl::create

[Jarred-Sumner]

What does this PR do?

Prevent synchronous finalizers from running in ExternalStringImpl::create

How did you verify your code works?

[robobun]

^{Updated 5:12 PM PT - Apr 9th, 2025}

✅ @Jarred-Sumner, your commit 1c3b477081134917699a702a2eb13b7b6cf734a0 passed in Build #14747! 🎉

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🧪   try this PR locally:

bunx bun-pr 18914

[190n]

may need to be rewritten again after #18767, but this is fine for now (on that PR once i finish fixing finalizers i will make sure all existing APIs with finalizers route through whatever i come up with)