Directory navigation keys in filepath completion

Background

When cd-ing to a nested directory (or selecting a deeply nested file), you need to type "enter tab enter tab enter tab..." to repeatedly select a folder and then re-start the filepath completion within that folder.

e.g. to cd to a/b/c/d, you would do

e.g. cd a/<tab><enter to select b><tab to list directories in b><enter to select c><tab to list directories in c><enter to select d>

Desired solution

Once in interactive completion mode, pressing tab would cause the completion to enter the the hovered-over directory without ending interactive completion.

e.g. cd a/<tab to select b and list directories within b><tab to select c and list directories within c><enter to select d>

i.e.

cd <tab>
cd a/<tab>
cd a/b<tab>
cd a/b/c<tab>
cd a/b/c/d<enter>

(For simplicity I've omitted filtering/using arrow keys to choose between directories before pressing tab)

Additionally, pressing shift+tab could go in the opposite direction, i.e. setting the current completion to its parent directory. This could even go past the initial directory (i.e. to .., then ../.., etc)

e.g.

cd a/b/c<shift-tab>
cd a/b<shift-tab>
cd a<shift-tab>
cd ..<shift-tab>
cd ../..<enter>

References

You can somewhat emulate this behavior using external tools like fzf or broot, but such tools are more heavyweight (slower) than needed for this use case since they also search unrelated directories. Implementing this in reedline would also make it easier to complete filepath arguments without needing to set up custom keybinds with commandline and such.

Breaking changes

This shouldn't break users since, as far as I am aware, neither tab nor shift-tab are used currently within the filepath completion menu.

Start by tracing reedline's interactive filepath completion mode and its handling of Tab and Shift-Tab. Done means Tab enters the hovered directory without leaving completion, while Shift-Tab moves to its parent, including paths above the initial directory, with the described cd examples working.