wizard: separate standard/action items from the list items using separator

(blocked until this lands in vscode stable API)

https://github.com/microsoft/vscode/issues/74967

https://code.visualstudio.com/updates/v1_63#_proposed-extension-apis

> QuickPickItem separators
>
> This iteration we are introducing the ability for extensions to add separators (also known as "categories") to a Quick Pick via a proposed API `vscode.proposed.quickPickSeparators.d.ts`. This is useful for grouping items or just having a bit of space between items:

First verify that the VS Code QuickPickItem separators API from the linked VS Code issue and proposed API documentation is available in stable VS Code. Then locate the extension's Quick Pick construction and identify the standard/action items and list items; done means those groups are displayed with separators between them.

Written by the indexing model from the issue text.