Support optional "parent" debug session in "startDebugging" API

[weinand]

When starting new debug sessions via the vscode.debug.StartDebugging API, it should be possible to specify a parent debug session so that the VS Code debugging UI can show sessions as a tree.

The proposal adds a new optional argument parent at the end of the existing argument list for startDebugging:

export namespace debug {

  /**
   * Start debugging by using either a named launch or named compound configuration,
   * or by directly passing a [DebugConfiguration](#DebugConfiguration).
   * The named configurations are looked up in '.vscode/launch.json' found in the given folder.
   * Before debugging starts, all unsaved files are saved and the launch configurations are brought up-to-date.
   * Folder specific variables used in the configuration (e.g. '${workspaceFolder}') are resolved against the given folder.
   * @param folder The [workspace folder](#WorkspaceFolder) for looking up named configurations and resolving variables or `undefined` for a non-folder setup.
   * @param nameOrConfiguration Either the name of a debug or compound configuration or a [DebugConfiguration](#DebugConfiguration) object.
   * @param parent If specified the newly created debug session(s) are registered as "child" sessions under the "parent" debug session.
   * @return A thenable that resolves when debugging could be successfully started.
   */
   export function startDebugging(folder: WorkspaceFolder | undefined, nameOrConfiguration: string | DebugConfiguration, parent?: DebugSession): Thenable<boolean>;

}

One use case can be found here: #62972