Add direct node-fetch dependency

[peterbom]

Is your feature request related to a problem? Please describe.
We don't have a direct dependency on node-fetch.

This will become a problem if/when we remove the cross-fetch dependency (which is likely to happen during the fix for #552), which indirectly ensures we have the node-fetch library needed for downloads.

Ideally we would have added a direct reference to node-fetch as part of #567, but doing this caused a patch version bump to several @microsoft/applicationinsights packages that are transitive dependencies of @vscode/extension-telemetry, which unearthed a bug that causes the extension to fail to load.

When the above bug is fixed, we can remove the @types/node-fetch dependency and add node-fetch directly.