Lf: Make variable containing files in the current directory in the order that they're being displayed

[mvrozanti]

This would solve the following use case:

Suppose you open a picture and you want to go to the next in the order that lf is displaying. We currently aren't really able to do that. But if we have this variable it would be possible.

A less desirable alternative solution would be to make a variable that contains the current sort type.

[mvrozanti]

Something like https://github.com/gokcehan/lf/pull/424

[gokcehan]

@mvrozanti You can use invert selection and $fs and $fx should contain the files in the displayed order.