Add threads to the ls command in order to increase performance in some circumstances (#13836)

# Description

This PR tries to allow the `ls` command to use multiple threads if so
specified. The reason why you'd want to use threads is if you notice
`ls` taking a long time. The one place I see that happening is from WSL.

I'm not sure how real-world this test is but you can see that this
simple `ls` of a folder with length takes a while 9366 ms. I've run this
test many times and it ranges from about 15 seconds to about 10 seconds.
But with the `--threads` parameter, it takes less time, 2744ms in this
screenshot.

![image](https://github.com/user-attachments/assets/e5c4afa2-7837-4437-8e6e-5d4bc3894ae1)

The only way forward I could find was to _always_ use threading and
adjust the number of threads based on if the user provides a flag. That
seemed the easiest way to do it after applying @devyn's interleave
advice.

No feelings hurt if this doesn't land. It's more of an experiment but I
think it has potential.

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crates/nu-cli/tests/completions/mod.rs

@@ -942,7 +942,7 @@ fn flag_completions() {
     // Test completions for the 'ls' flags
     let suggestions = completer.complete("ls -", 4);
 
-    assert_eq!(16, suggestions.len());
+    assert_eq!(18, suggestions.len());
 
     let expected: Vec<String> = vec![
         "--all".into(),
@@ -953,6 +953,7 @@ fn flag_completions() {
         "--long".into(),
         "--mime-type".into(),
         "--short-names".into(),
+        "--threads".into(),
         "-D".into(),
         "-a".into(),
         "-d".into(),
@@ -961,6 +962,7 @@ fn flag_completions() {
         "-l".into(),
         "-m".into(),
         "-s".into(),
+        "-t".into(),
     ];
 
     // Match results