Support tab completion / custom key handlers in QuickPick

<!-- ⚠️⚠️ 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. --> In https://github.com/jit-y/vscode-advanced-open-file, we create a QuickPick with a full file path that live-updates as you type with files matching the path. I'd like to add tab-completion so that, when you hit tab with the dialog open, it autocompletes the path if there's only one option visible, and if there are multiple matches, autocompletes up to the common prefix between them. API-wise, it'd be nice to be able to register a keydown event handler or something for when the text input is focused so that we can read whether the user hit Tab and decide how to respond. Currently, tab switches focus to the list of choices, which seems redundant to me since the choice list responds to input even without focus (arrow keys and enter still work). This might be an accessibility thing, though? Thank you!