feat(list_menu): add DescriptionPosition to control description place…

[a-rookie-of-C-language]

Add a DescriptionPosition enum that lets callers choose whether the
completion description is rendered before or after the value in a
ListMenu row.

Previously the position was hardcoded as (description) value.
The new default keeps that behaviour (Before) for backwards
compatibility; setting After renders value description instead,
which is more natural for flag/option completions where the value
(e.g. --regexp) should visually come first.

Changes:

• DescriptionPosition enum with Before (default) and After variants
• ListMenu::description_position field (default: Before)
• ListMenu::with_description_position() builder method
• create_string() branches on self.description_position
• Re-export DescriptionPosition from menu/mod.rs and lib.rs

This mirrors the existing IdeMenu::with_description_mode() pattern.