small, backwards compatible enhancements to std (#14763)

# Description
Small, backwards compatible enhancements to the standard library.

# User-Facing Changes

- changed `iter find`, `iter find-index`: Only consume the input stream
up to the first match.
- added `log set-level`: a small convenience command for setting the log
level
- added `$null_device`: `null-device` as a const variable, would allow
conditional sourcing if #13872 is fixed

# Tests + Formatting

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

crates/nu-std/std/util/mod.nu

@@ -110,15 +110,18 @@ export def repeat [
     1..$n | each { $item }
 }
 
+# null device file
+export const null_device = if $nu.os-info.name == "windows" {
+	'\\.\NUL'
+} else {
+	'/dev/null'
+}
+
 # return a null device file.
 #
 # # Examples
 #     run a command and ignore it's stderr output
 #     > cat xxx.txt e> (null-device)
 export def null-device []: nothing -> path {
-    if ($nu.os-info.name | str downcase) == "windows" {
-        '\\.\NUL'
-    } else {
-        "/dev/null"
-    }
+    $null_device
 }