[VSCode] TypeSpec Extension should leverage each emitter to detect the dependency missing and provide suggestion on next step

[lirenhe]

Clear and concise description of the problem

Here are a couple of actions needed:

1. Design the format and the contract of env check result should return.
2. Define the command e.g. 'tsp compile -check' that could be used by the extension.
3. Integrate the result into the extension and suggest the follow up action for each language. cc @RodgeFu, @lmazuel

Checklist

• [x] Follow our Code of Conduct
• [x] Read the docs.
• [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.