Rename IoStream to OutDest (#12433)

# Description
I spent a while trying to come up with a good name for what is currently
`IoStream`. Looking back, this name is not the best, because it:
1. Implies that it is a stream, when it all it really does is specify
the output destination for a stream/pipeline.
2. Implies that it handles input and output, when it really only handles
output.

So, this PR renames `IoStream` to `OutDest` instead, which should be
more clear.

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

@@ -25,7 +25,7 @@ impl NuCompleter {
     pub fn new(engine_state: Arc<EngineState>, stack: Stack) -> Self {
         Self {
             engine_state,
-            stack: stack.reset_stdio().capture(),
+            stack: stack.reset_out_dest().capture(),
         }
     }
 

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

@@ -24,7 +24,7 @@ impl CustomCompletion {
     pub fn new(engine_state: Arc<EngineState>, stack: Stack, decl_id: usize, line: String) -> Self {
         Self {
             engine_state,
-            stack: stack.reset_stdio().capture(),
+            stack: stack.reset_out_dest().capture(),
             decl_id,
             line,
             sort_by: SortBy::None,

crates/nu-cli/src/menus/menu_completions.rs

@@ -28,7 +28,7 @@ impl NuMenuCompleter {
         Self {
             block_id,
             span,
-            stack: stack.reset_stdio().capture(),
+            stack: stack.reset_out_dest().capture(),
             engine_state,
             only_buffer_difference,
         }