API for custom editors handling some menu actions

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Considering a custom editor that does not necessarily deal with text, it'd improve the user experience if we could have the ability to provide a customized action when menu items are triggered, such as `Edit->Find` and `Selection->Select All`. Please also consider adding to this API the ability to handle cut/copy/paste. Although we could use events such as the [copy event](https://developer.mozilla.org/en-US/docs/Web/API/Element/copy_event) for detecting this operation, a unified API for menu items relevant to custom editors seems more appropriate. /cc @mjbvz