std: add cross platform null-device name (nushell#11070)

# Description
We have meet a discord discussion about cross platform `null-device`:
https://discord.com/channels/601130461678272522/988303282931912704/1165966467095875624

I want the same feature too...And I think it's good enough to add it
into `nu-std`.

Different to https://github.com/nushell/nu_scripts/pull/649/files, I
think named it to `null-device`(the name comes from
[wiki](https://en.wikipedia.org/wiki/Null_device)) is better than
`null-stream`.

crates/nu-std/std/mod.nu

@@ -335,3 +335,16 @@ export def repeat [
 
     1..$n | each { $item }
 }
+
+# return a null device file.
+#
+# # Examples
+#     run a command and ignore it's stderr output
+#     > cat xxx.txt e> (null-device)
+export def null-device []: nothing -> path {
+    if ($nu.os-info.name | str downcase) == "windows" {
+        '\\.\NUL'
+    } else {
+        "/dev/null"
+    }
+}