che: VS Code Extension API does not work across Extension Host Boundaries

A VS Code extension can provide API to other extensions by returning an API object from their activate function (see https://code.visualstudio.com/api/references/vscode-api#extensions.getExtension). This works fine when the extensions run in the same process, but when the extensions run in their own sidecar, I think this will not work. This is relevant to Che: some very basic extensions like yaml support (https://github.com/redhat-developer/vscode-yaml) use this facility to communicate with other extensions like tekton support.