Set the capacity of the Vec used in gather_captures() to the number of captures expected (#13339)

# Description

Just more efficient allocation during `Stack::gather_captures()` so that
we don't have to grow the `Vec` needlessly.

# User-Facing Changes
Slightly better performance.

crates/nu-protocol/src/engine/stack.rs

@@ -277,7 +277,7 @@ impl Stack {
     }
 
     pub fn gather_captures(&self, engine_state: &EngineState, captures: &[VarId]) -> Stack {
-        let mut vars = vec![];
+        let mut vars = Vec::with_capacity(captures.len());
 
         let fake_span = Span::new(0, 0);