Add support for 'sim extensions'

[jwunderl]

It'd be nice to add a command to plug in a file with a message listener that'd send / receive along this channel: https://github.com/microsoft/pxt-common-packages/blob/master/libs/base/controlmessage.ts to make it easier to make a program that communicates with the outer iframe / can actually work with browser apis (e.g. fetch data from outside source, etc)

Intention already should be there in mkc with https://github.com/microsoft/pxt-mkc/blob/master/packages/makecode-core/simloader/custom.js / https://github.com/microsoft/pxt-arcade/blob/master/libs/template/assets/js/custom.js, so mainly hooking into that & testing