File picker prompt rework and custom input component

[mcobzarenco]

• Add a custom input component, rather than relying on zi's input. This change enables reusing zee's text editing functions and makes the editing prompt input behave similar to text editing in a buffer.
• Refactor file pickers to use a newly added custom input component. New text editing bindings were introduced to match buffers. A number of edge cases and bugs were fixed, e.g. when editing an empty path.