Make Value::columns return slice instead of cloned Vec (#9927)

# Description
This PR changes `Value::columns` to return a slice of columns instead of
cloning said columns. If the caller needs an owned copy, they can use
`slice::to_vec` or the like. This eliminates unnecessary Vec clones
(e.g., in `update cells`).

# User-Facing Changes
Breaking change for `nu_protocol` API.

crates/nu-protocol/src/value/mod.rs

@@ -1732,10 +1732,10 @@ impl Value {
         matches!(self, Value::Bool { val: false, .. })
     }
 
-    pub fn columns(&self) -> Vec<String> {
+    pub fn columns(&self) -> &[String] {
         match self {
-            Value::Record { cols, .. } => cols.clone(),
-            _ => vec![],
+            Value::Record { cols, .. } => cols,
+            _ => &[],
         }
     }