Convert PluginFailedToEncode to named fields (#11125)

# Description

Part of #10700

# User-Facing Changes

None

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

N/A

crates/nu-plugin/src/protocol/mod.rs

@@ -88,7 +88,7 @@ impl From<ShellError> for LabeledError {
                 msg,
                 span: None,
             },
-            ShellError::PluginFailedToEncode(msg) => LabeledError {
+            ShellError::PluginFailedToEncode { msg } => LabeledError {
                 label: "Plugin failed to encode".into(),
                 msg,
                 span: None,