API Proposal: Support Custom Native Floating Modal Dialogs for Complex Data Workflows

### Problem Statement & Use Case We are migrating complex developer tooling from Eclipse RCP to VS Code extensions. We have encountered a major layout and workflow blocker regarding advanced user input and data validation. In Eclipse RCP, the `Dialog` architecture allows launching modal frames to handle heavy data orchestration (e.g., database schema mapping, multi-row environment setups, and complex multi-field forms) without breaking the user's active editor split layouts. Currently, VS Code offers no viable alternative for dense, non-linear data collection: * `showInformationMessage(..., { modal: true })`: Highly restrictive. It only displays static strings and buttons; it cannot collect complex user input. * `showQuickPick` / Multi-step InputBox: Built for linear selection. It completely fails when workflows require structural matrices, multi-column tables, cross-field validation, or complex form controls. * `WebviewPanel` (Editor Tabs): Opening a full-sized tab for a transient 10-second task disorients the user's layout, breaks active editor splits, and introduces a messy tab lifecycle management overhead. ### Proposed Solution We propose an API extension such as `vscode.window.createExtensionDialog`—that allows extensions to spawn a temporary modal overlay surface over the workbench: * **True Modality:** Blocks interaction with the underlying workbench layout until explicitly submitted or cancelled to guarantee data sequence integrity. * **Floating & Centered:** Container floats centered over the active VS Code window (similar to native OS message boxes). * **Webview Driven:** Powered by standard sandboxed Webview HTML/JS, allowing developers to build custom data grids, checkboxes, validation logic, and input fields safely. * **Transient Lifecycle:** Exists entirely outside the editor tab bar, managed explicitly by the extension backend or user actions (`Esc` / Close buttons).