Filter IntelliSense completions by declaration type

[majastrz]

We should not be suggesting the concat() function if the contextual/declared type at the current position is not string or one of its subtypes.

[majastrz]

Thought about it some more...

I think the safest way would be to filter out functions that return simple types (string, bool, int) when the contextual type of the property isn't compatible. We should still allow functions that return objects because when they user is assigning location of a resource, they would want to see the completion for resourceGroup() so they can use . to access the location property of the RG.