Both tasks ShellTask and TerminalTask use a happy mix of readonly, xyz | undefined type declarations, and non JS-ish defaults. I'd recommend to clarify things, e.g

• Use question mark for types that can be undefined, e.g CompletionItem#insertText
• Don't use readonly the model is anyway that VS Code created a disconnected copy of that data providers return
• Don't use empty defaults like an empty array or empty object. The JavaScript programmer expects undefined