FR: Get type checking to match val.town editor

[saolsen]

Not sure how much work this would be but it would be cool if the type checking matched the val.town editor.
I think it'd require some integration with the deno extension. I tried just using it but it doesn't work out of the box, it complains about not being able to import some modules and I think it doesn't recognize the vals as being part of a deno project or something.

[pomdtr]

I did not find a way to hook the deno extension to a virtual file system, so I'm not sure we can leverage it.