Add option to scan deeper for CDK projects in CDK Explorer

Ideally you'd be able to select a depth, but "shallow" (current behavior: root folder and its subfolders only) and "deep" (scan entire folder hierarchy under root) might be sufficient and easier to use.

Start by locating CDK Explorer's current scan of the root folder and its immediate subfolders, then trace how scan options are exposed in the VS Code extension. Done means users can choose shallow or deep scanning and the selected behavior is covered by the relevant existing tests, if any.

Written by the indexing model from the issue text.