add iter module to standard library (#8899)

# Description
 
this pr introduces an `iter` module
to the standard library.

the module is aimed at extending the filter commands.

crates/nu-std/lib/mod.nu

@@ -5,6 +5,7 @@ export-env {
     use dirs *
 }
 export use help *
+export use iter *
 export use log *
 export use testing *
 export use xml *