support typed property values in Context

Created by: sqs

This allows the context (which is used for when expressions and other expressions in an extension manifest) to contain properties whose value is a TypeScript type other than the primitive types (or Context itself). This is useful when, for example, you have a TextDocumentPositionParams value that is the value of a context property; using Context<TextDocumentPositionParams> makes it typecheck.

There is no user-facing behavior change. This is just a change to types, not impl code.

Part of #1313 but useful by itself.