Design 'app model' file

We want to keep some workspace-local metadata that maps the user's code to deployed Lambda functions (and serverless apps). This is the 'app model' file concept that's been floated around and is similar in concept to the Cloud9 .application.json file.

This task is to design what we'll do in VSCode. Output will be a design doc for discussion and one or more tasks for implementation.

* Can we re-use the Cloud9 file (and perhaps the jetBrains version, if it should exist)? How do we keep in sync with them if so?
* Do we need our own file? If so, what's the format and what's the round trip story with Cloud9?

I'm sure other issues and considerations will come forward when we start to think this through more deeply!

Start by reviewing the issue's app model concept and the Cloud9 .application.json format, then investigate whether a JetBrains equivalent exists. Compare reuse versus a VSCode-specific file, including synchronization and Cloud9 round-trip concerns, and document the proposed design with follow-up implementation tasks.

Written by the indexing model from the issue text.