simplify extension API context impl

Created by: sqs

The getComputedContextProperty function is called to provide the values for extension context keys such as resource.uri (for extensions to define actions that, when invoked, call a function with the URI of the currently visible document).

Now, instead of getComputedContextProperty parsing the requested key (such as resource.uri) and running custom code to return each key's value, it constructs a JavaScript object with all possible keys and just indexes into that object.

Finally, the getComputedContextProperty is renamed to computeContext. It now always returns the entire computed context data as a map, instead of computing the value per-key. This is a simplification because it enforces that there must be no special per-key computation; the context can be treated as a map instead of as an interface with an arbitrary {get(key:string): any} function.

This is not expected to have any impact on extensions. It does slightly change the behavior, such as context expressions that used to return null now return undefined. I will search existing extension code for potential backcompat problems and will test it more before merging.