Keybinding when clause has context for extension is enabled/installed

<!-- ⚠️⚠️ 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. --> Rust-analyzer's current technique for auto inserting comments is with the following custom keybinding: ```json { "key": "Enter", "command": "rust-analyzer.onEnter", "when": "editorTextFocus && !suggestWidgetVisible && editorLangId == rust" } ``` (from https://github.com/rust-analyzer/rust-analyzer/issues/4980) The problem is that if the `rust-analyzer` extension is not installed or is disabled, pressing enter in the editor (when all the *when* conditions are met) throws a error with `⚠ command 'rust-analyzer.onEnter' not found` **and** prevents the default return insertion. Currently there isn't a way to prevent this apart from remove the keybinding temporarily. This could be solved by adding a new context object. I looked at the [when contexts](https://code.visualstudio.com/api/references/when-clause-contexts#available-contexts) and there isn't a extension one available in editor? Maybe ` "when": "extensions.available.has('rust-analyzer') && ..."` ...? Alternatively could this be done automatically, could vscode see that the extension is not available and ignore this keybinding?