Feature Request: `enablement` property on `MessageItem`s and `QuickPickItem`s

<!-- ⚠️⚠️ 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. --> Because our extensions are supporting multiple environments, it would be nice to have an `enablement` property on the `vscode.MessageItem` and `vscode.QuickPickItem` objects. Similar to context commands, it would be great if they could be grayed out and disabled. Examples of enablement would be `isWeb`, `isVirtualEnvironment`, any condtional statement, etc. The `package.json` has this implementation when you define the commands in the manifest. It would be perfect if that behavior could be duplicated because as of now, our extensions have to hide quick pick items that don't work in a web environment, but that can get confusing for users when options that were available in desktop suddenly disappear.