Fix insiders build fails to get extension list

[sonhanguyen]

Replace the unreliable check for packageJSON.__metadata (not present in insiders) with extensionPath.
This is to address #428

[shanalikhan]

This change fix the insiders, will current stable VS Code work with this ?

[sonhanguyen]

@shanalikhan I tested both, seem to work.

[sonhanguyen]

I'm not happy with hardcoding the extension directory (.vscode(-insiders)?) though. There is a flag (--extensionHomePath) that overrides this but I could not find the api to get it.

[shanalikhan]

Yes, there are many things need to done from API side.
I'm looking for contributors so i can guide them and make this extension more smooth and with rich features.
Let me know if you are willing to work with other issues.

Thanks,
P.s will release new version tonight