Include way of listing all available tables

[patrickdevivo]

As mentioned in: https://github.com/askgitdev/askgit/pull/207#issuecomment-956167714, it would be valuable to have a way to list the available tables (table-valued-functions) provided by askgit and have the method documented. This PRAGMA could be a simple option, but it mixes tables provided by this project and other modules (the JSON functions for example).

It may be worth doing this at the application level and offering a subcommand or "built-in" view/table that lists the askgit tables.