Add new 'context' parameter to EvaluateRequest

[daviwil]

The VS Code team has added a new 'context' parameter to the EvaluateRequest which allows us to determine whether the request comes from the watch window or the REPL. This makes it easier to know whether a request should be evaluated against the current variables list or executed against the runspace.

microsoft/vscode#252

We'll need to add this parameter at some point before the next VS Code release.

[daviwil]

Looked into this briefly a couple days ago, would require bringing back some logic to walk a variable expression to gather its contents. I'm going to hold off on this temporarily until I have more time to look at it.