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-command/src/system/run_external.rs

@@ -1,6 +1,6 @@
 use nu_cmd_base::hook::eval_hook;
 use nu_engine::{command_prelude::*, env_to_strings, get_eval_expression};
-use nu_protocol::{ast::Expr, did_you_mean, IoStream, ListStream, NuGlob, RawStream};
+use nu_protocol::{ast::Expr, did_you_mean, ListStream, NuGlob, OutDest, RawStream};
 use nu_system::ForegroundChild;
 use nu_utils::IgnoreCaseExt;
 use os_pipe::PipeReader;
@@ -225,8 +225,8 @@ pub struct ExternalCommand {
     pub name: Spanned<String>,
     pub args: Vec<Spanned<String>>,
     pub arg_keep_raw: Vec<bool>,
-    pub out: IoStream,
-    pub err: IoStream,
+    pub out: OutDest,
+    pub err: OutDest,
     pub env_vars: HashMap<String, String>,
 }
 
@@ -523,7 +523,7 @@ impl ExternalCommand {
                         RawStream::new(Box::new(ByteLines::new(err)), ctrlc.clone(), head, None)
                     });
 
-                    if matches!(self.err, IoStream::Pipe) {
+                    if matches!(self.err, OutDest::Pipe) {
                         (stderr, stdout)
                     } else {
                         (stdout, stderr)
@@ -634,7 +634,7 @@ impl ExternalCommand {
 
         // If the external is not the last command, its output will get piped
         // either as a string or binary
-        let reader = if matches!(self.out, IoStream::Pipe) && matches!(self.err, IoStream::Pipe) {
+        let reader = if matches!(self.out, OutDest::Pipe) && matches!(self.err, OutDest::Pipe) {
             let (reader, writer) = os_pipe::pipe()?;
             let writer_clone = writer.try_clone()?;
             process.stdout(writer);