Remove some unnecessary static Vecs (#11947)

Avoid unnecessary allocations or larger iterator structs

- Turn static `Vec`s into arrays when possible
- Use `std::iter::once`/`empty` where applicable
- Use `bool::then_some` in `detect column` `.chain`
- Drop in the bucket: de-vec-ing tests

crates/nu-cli/src/completions/command_completions.rs

@@ -253,7 +253,7 @@ mod command_completions_tests {
 
     #[test]
     fn test_find_non_whitespace_index() {
-        let commands = vec![
+        let commands = [
             ("    hello", 4),
             ("sudo ", 0),
             (" 	sudo ", 2),
@@ -273,7 +273,7 @@ mod command_completions_tests {
 
     #[test]
     fn test_is_last_command_passthrough() {
-        let commands = vec![
+        let commands = [
             ("    hello", false),
             ("    sudo ", true),
             ("sudo ", true),

crates/nu-cli/src/completions/completer.rs

@@ -564,7 +564,7 @@ mod completer_tests {
         );
 
         let mut completer = NuCompleter::new(engine_state.into(), Stack::new());
-        let dataset = vec![
+        let dataset = [
             ("sudo", false, "", Vec::new()),
             ("sudo l", true, "l", vec!["ls", "let", "lines", "loop"]),
             (" sudo", false, "", Vec::new()),

crates/nu-cli/src/reedline_config.rs

@@ -84,7 +84,7 @@ pub(crate) fn add_menus(
     }
 
     // Checking if the default menus have been added from the config file
-    let default_menus = vec![
+    let default_menus = [
         ("completion_menu", DEFAULT_COMPLETION_MENU),
         ("history_menu", DEFAULT_HISTORY_MENU),
         ("help_menu", DEFAULT_HELP_MENU),