Better handling of dependent extension's "enablement"

This extension (`vscode-trace-server`) is a companion to extension `eclipse-cdt.vscode-trace-extension` (`Trace Viewer for VSCode`) and depends on it.

At activation, this extension obtains a reference to the trace viewer's exported API object, and uses it to provide helper functionality. This work well normally, but there may be a corner-case that we should handle better: if/when the `Trace Viewer` extension becomes disabled, we probably should not use its API object any more, and if/when it becomes enabled again, we should obtain a fresh API object and redo the activation steps.

One scenario where the above might happen (to be confirmed) is when the `Trace Viewer` extension is updated to a new version.

As a starting point, here is how it seems the built-in `vscode.github` extension monitors the "enablement" of `vscode.git-base` it depends-on, and "refreshes" its API object when needed: