Port xplr.fn.builtin.fmt_general_selection_item over to Rust

[Arijit Basu]

See Lua implementation:

xplr/src/init.lua

Line 2916 in f3a2d51

 xplr.fn.builtin.fmt_general_selection_item = function(n) 

Default to the Rust implementation if xplr.config.general.selection.item.format = nil (default value)

xplr/src/init.lua

Line 252 in f3a2d51

 xplr.config.general.selection.item.format = "builtin.fmt_general_selection_item" 

Subtask of #641