Add an `onExtensionActivated` activation event

I have an extension (https://github.com/intersystems/language-server) that depends on another one (https://github.com/intersystems-community/vscode-objectscript). Instead of having to duplicate activation events between them to ensure that the dependent extension always activates when the main extension does, it would be nice to have a single activation event `onExtensionActivated:intersystems-community.vscode-objectscript`.