feature: set up tab completion for positional arguments

[rnag]

Description

It would be nice to set up tab completion for the first positional argument after rx, similar to how it auto completes when you type cd to switch into a directory and press tab after entering a partial match, for example.

The rx command in this case should offer auto-completion suggestions based on the list of examples for the current Cargo project.

For example if there is a single example in a project called hello_world.rs, when I enter hello it should auto complete to hello_world. If there is more than one example that starts with the entered term, it should offer all possible examples that match the query.