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/tests/test_iter.nu

@@ -0,0 +1,50 @@
+use std *
+
+export def test_iter_find [] {
+    let hastack1 = [1 2 3 4 5 6 7]
+    let hastack2 = [nushell rust shell iter std]
+    let hastack3 = [nu 69 2023-04-20 "std"]
+
+    let res = ($hastack1 | iter find {|it| $it mod 2 == 0})
+    assert equal $res 2
+
+    let res = ($hastack2 | iter find {|it| $it starts-with 's'})
+    assert equal $res 'shell'
+
+    let res = ($hastack2 | iter find {|it| ($it | length) == 50})
+    assert equal $res null
+
+    let res = ($hastack3 | iter find {|it| (it | describe) == filesize})
+    assert equal $res null
+}
+
+export def test_iter_intersperse [] {
+    let res = ([1 2 3 4] | iter intersperse 0)
+    assert equal $res [1 0 2 0 3 0 4]
+
+    let res = ([] | iter intersperse x)
+    assert equal $res []
+
+    let res = ([1] | iter intersperse 5)
+    assert equal $res [1]
+
+    let res = ([a b c d e] | iter intersperse 5)
+    assert equal $res [a 5 b 5 c 5 d 5 e]
+
+    let res = (1..4 | iter intersperse 0)
+    assert equal $res [1 0 2 0 3 0 4]
+
+    let res = (4 | iter intersperse 1)
+    assert equal $res [4]
+}
+
+export def test_iter_scan [] {
+    let scanned = ([1 2 3] | iter scan 0 {|x, y| $x + $y} -n)
+    assert equal $scanned [1, 3, 6]
+
+    let scanned = ([1 2 3] | iter scan 0 {|x, y| $x + $y})
+    assert equal $scanned [0, 1, 3, 6]
+
+    let scanned = ([a b c d] | iter scan "" {|x, y| [$x, $y] | str join} -n)
+    assert equal $scanned ["a" "ab" "abc" "abcd"]
+}