DAP: Configure child process stderr as piped

By default this is `Stdio::inherit` which sends stderr from the child
process to Helix. Instead we should use `Stdio::piped` which allows us
to read the piped output.

We can also expect that the stderr opens now (it should similarly to
stdout), so that we always start a reader for stderr like the LSP
client.

helix-dap/src/client.rs

@@ -119,6 +119,7 @@ impl Client {
             .args(args)
             .stdin(Stdio::piped())
             .stdout(Stdio::piped())
+            .stderr(Stdio::piped())
             // make sure the process is reaped on drop
             .kill_on_drop(true)
             .spawn();
@@ -128,16 +129,12 @@ impl Client {
         // TODO: do we need bufreader/writer here? or do we use async wrappers on unblock?
         let writer = BufWriter::new(process.stdin.take().expect("Failed to open stdin"));
         let reader = BufReader::new(process.stdout.take().expect("Failed to open stdout"));
-        let errors = process.stderr.take().map(BufReader::new);
+        let stderr = BufReader::new(process.stderr.take().expect("Failed to open stderr"));
 
         Self::streams(
             Box::new(BufReader::new(reader)),
             Box::new(writer),
-            // errors.map(|errors| Box::new(BufReader::new(errors))),
+            Some(Box::new(stderr)),
-            match errors {
-                Some(errors) => Some(Box::new(BufReader::new(errors))),
-                None => None,
-            },
             id,
             Some(process),
         )