Add support for intellisense in experimental debugger

[DonJayamanne]

• Enable tests in capabilities.test.ts

[brettcannon]

Support how?

[DonJayamanne]

Debugger protocol supports this today. Its up to the debugger adapter to return the completion items.
We could either perform something as dirty as dir on the objects (I believe PTVSD does this) or use Jedi (other IDEs seem to use jedi in the debugger to provide intellisense).