Enable use of Tools.LoadScript in a WebWorker

[wmurphyrd]

Fixes #12882

• Checks if LoadScript is called from a WebWorker context and, if so, uses importScripts instead of HTMLScriptElement to load the script.
• Ensures LoadScript always resolves with either onSuccess or onError

Use case: using ImportMeshAsync in a WebWorker for meshes requiring MeshoptCompression

RE the typings: there's apparently no clean solution as TypeScript is extremely not a fan of code designed to run in both DOM and WebWorker, these hideous double type assertions seemed less bad than falsely declaring importScripts as available on the Window interface

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
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[sebavan]

GG !!!! thanks a lot

[sebavan]

Just a tiny format issue and good to go :-)

[deltakosh]

you rock! thanks a lot

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[azure-pipelines]

Snapshot stored with reference name:
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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12884/merge/index.html

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