[playground] QoL use webpack to bundle monaco-editor's loader

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This lets us load playground locally, entirely offline (e.g. on an airplane, 
when a cdn is down, at ReactAdvanced, etc) 

Before: 

See external script link 

<img width="691" alt="image" 
src="https://github.com/facebook/react-forget/assets/34200447/85500e86-c1a5-4dff-bdcd-58db0d6fd453"> 

Now: 

No external script 

<img width="476" alt="image" 
src="https://github.com/facebook/react-forget/assets/34200447/fdcbea09-0b5f-42e3-9310-35d8d1d726b7">