Lint redundant explicit activation events in package.json

[joyceerhl]

Following the addition of implicit activation events in the November release, we'd like to provide lint warnings via the extension-editing extension when an extension declares activation events which are already implied by its declared contributions.

[joyceerhl]

cc @jrieken as a heads up--we previously discussed this in #166485 and at the time agreed that a simple warning for the reserved/internal onNotebookSerializer event was sufficient. We've since updated the docs and extension samples to mention that you don't need to explicitly list many events anymore. In discussion with Kai, the next step to round out this work and make extension authors aware of the new behavior is to provide lint warnings for all redundant events in the extension-editing extension. Let me know if you have any questions or concerns.