pxt-arcade: Unable to build the project due to the private pxt-arcade-sim dependency

Describe the bug As a developer external to Microsoft organization I am no longer able to build and run pxt-arcade due to the private npm dependency introduced in https://github.com/Microsoft/pxt-arcade/pull/684.

To Reproduce Steps to reproduce the behavior:

1. I am following steps described in https://github.com/Microsoft/pxt-arcade#setup

	git clone https://github.com/Microsoft/pxt-arcade.git 
	cd pxt-arcade
	npm install

1. I got the error:

3573 verbose stack Error: exited with error code: 128
3573 verbose stack     at ChildProcess.<anonymous> (C:\Program Files\nodejs\node_modules\npm\node_modules\pacote\lib\util\finished.js:12:19)
3573 verbose stack     at ChildProcess.emit (events.js:182:13)
3573 verbose stack     at maybeClose (internal/child_process.js:962:16)
3573 verbose stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:251:5)
3574 verbose cwd C:\java\projects\pxt-arcade
3575 verbose Windows_NT 10.0.17134
3576 verbose argv "C:\\Program Files\\nodejs\\node.exe" "C:\\Program Files\\nodejs\\node_modules\\npm\\bin\\npm-cli.js" "install"
3577 verbose node v10.15.0
3578 verbose npm  v6.4.1
3579 error Error while executing:
3579 error C:\Program Files\Git\mingw64\bin\git.EXE ls-remote -h -t https://github.com/Microsoft/pxt-arcade-sim.git
3579 error
3579 error remote: Invalid username or password.
3579 error fatal: Authentication failed for 'https://github.com/Microsoft/pxt-arcade-sim.git/'
3579 error
3579 error exited with error code: 128
3580 verbose exit [ 1, true ]

Expected behavior I think npm install should work for people outside of Microsoft organization as well. At least it was working prior to the changes introduced in https://github.com/Microsoft/pxt-arcade/pull/684.

Desktop (please complete the following information):

• OS: Windows 10
• Node v10.15.0
• npm 6.4.1
• Git Bash

Additional context I think the root cause is that pxt-arcade-sim dependency mentioned in package.json resides in Microsoft private repository (https://github.com/Microsoft/pxt-arcade-sim). To fix the issue please either publish it to the public NPM registry or make pxt-arcade-sim repository public.

P.S. It would be really great if you could also publish pxt-arcade to NPM registry as well (e.g. how it was done for pxt-microbit), so 3rd party developers no longer need to build pxt-arcade locally prior to building their pxt game packages.