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-explore/src/nu_common/command.rs

@@ -3,7 +3,7 @@ use nu_parser::parse;
 use nu_protocol::{
     debugger::WithoutDebug,
     engine::{EngineState, Redirection, Stack, StateWorkingSet},
-    IoStream, PipelineData, ShellError, Value,
+    OutDest, PipelineData, ShellError, Value,
 };
 use std::sync::Arc;
 
@@ -96,8 +96,8 @@ fn eval_source2(
     }
 
     let stack = &mut stack.push_redirection(
-        Some(Redirection::Pipe(IoStream::Capture)),
-        Some(Redirection::Pipe(IoStream::Capture)),
+        Some(Redirection::Pipe(OutDest::Capture)),
+        Some(Redirection::Pipe(OutDest::Capture)),
     );
     eval_block::<WithoutDebug>(engine_state, stack, &block, input)
 }