Improve fzf interface for selecting trigger by line numbers

If magic-trace gains ability to trigger by line numbers from #241, it would be nice to have a way to fzf on files. Perhaps files could be included in the fzf and if you select a file, magic-trace then asks for you to select a line number.

This could also be conditionally used perhaps only when someone passes `-trigger line:?` or could unconditionally include files.

Start by reviewing issue #241 and the existing trigger-selection flow in magic-trace. Define whether file selection is conditional on `-trigger line:?` or always shown, then make completion support choosing a file followed by a line number and verify the resulting fzf interaction.

Written by the indexing model from the issue text.