Add tab file name expansion (and file browsing) to @autocomplete

What feature would you like to see?

• Add tab name expansion mode to @autocomplete. In the tab expansion mode, rather than globally searching for all matching files in cwd, paths are expanded (similar to shell path expansion)
• The file search popup in this mode displays matching results (effectively directory listing or subset) and supports drill-down when pressing Enter on a directory name
• The mode is complementary to the default @query search, and is entered upon the first tab press once in the @ mode (whether there is a query or not)

Are you interested in implementing this feature?

Yes, have proof of concept branch that works.

Additional information

Need to standardize behavior of paths with spaces (which, generally, cannot be part of the existing global @query but can be created through expansion - WIP)

The branch with the feature can be seen/tested here: https://github.com/mrx3000/codex/tree/file_search_tab_expansion