Intellisense for consuming step outputs

[itowlson]

When a prior step has an output, that output should be available as an completion item for values in subsequent steps (bundle.output.<stepname>.<outputname> I think).

[carolynvs]

Steps don't have unique names, just descriptions, but outputs do have unique names per bundle. So we can do bundle.output.outputname.

So it would look something like this (abbreviated for clarity):

install:
- helm:
     outputs:
     - name: output1
- exec:
     command: echo
     args:
     - "source: bundle.outputs.output1"