Populate '@' menu dynamically

## Problem
In the AmazonQ VSCode extension, typing '@' in chat or using the Pin Context menu makes features available including @workspace, @file, @folder, etc. However, this only occurs after the entire workspace is finished indexing, and not a moment sooner. Until then, typing '@' in chat or attempting to use the Pin Context menu does nothing. For large workspaces, the user can be waiting for several minutes before they are able to interact with the extension features.

Other issue tickets which I suspect are related:
https://github.com/aws/aws-toolkit-vscode/issues/8185
https://github.com/aws/aws-toolkit-vscode/issues/7947
https://github.com/aws/aws-toolkit-vscode/issues/7987

## Feature Request
Do not make the user wait longer than necessary. Populate the Pin Context menu incrementally and dynamically, starting with the data most likely to be desired by the user. *First*, add the active file, the file open in the VSCode editor. Second, add the other files currently open in VSCode editor tabs. Third, move on to the parent directories of these files and their contents, recursively, up to the workspace folders. (And of course keep track, so as not to double-count anything.)

Start at the Amazon Q VSCode extension's Pin Context menu and '@' chat handling, then trace how availability waits for workspace indexing. Verify that the active editor file, other open tabs, parent directories, and workspace folders become available in that order before indexing completes, without duplicate entries.

Written by the indexing model from the issue text.