[stable8.5] Cherry-pick built sim JS changes

[aznhassan]

• Before compiling, try to fetch built js from backend (Before compiling, try to fetch built js from backend #9778)]
• Track js compilation time (Track js compilation time #9794)

Validation

• Ran these changes locally with pxt-arcade on the stable1.12 branch.
• Hit the http://localhost:3232/68336-13291-41085-56601 endpoint and confirmed the already compiled JS was downloaded.
• Confirmed the new perfMeasurement event was sent via the network tab.