refactor: enable context isolation

[dsanders11]

Stacked on top of #1452 so in draft mode until that lands.

Only a couple of changes needed:

• Had to push destructuring RunnableVersion up to the callers of window.ElectronFiddle functions - these are proxies since they're observable by MobX and you can't clone proxies across the context isolation interface
• Had to move app and monaco off of window.ElectronFiddle as they're set during initialization inside the renderer process and window.ElectronFiddle has read-only properties once context isolation is enabled

[coveralls]

coverage: 87.085%. remained the same when pulling 1b1680b on enable-context-isolation into 57b50d4 on main.

[felixrieseberg]

Thanks for your hard work!