Convert ShellError::AliasNotFound to named fields (#11118)

# 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-protocol/src/shell_error.rs

@@ -701,7 +701,10 @@ pub enum ShellError {
     /// The alias does not exist in the current scope. It might exist in another scope or overlay or be hidden.
     #[error("Alias not found")]
     #[diagnostic(code(nu::shell::alias_not_found))]
-    AliasNotFound(#[label("alias not found")] Span),
+    AliasNotFound {
+        #[label("alias not found")]
+        span: Span,
+    },
 
     /// Failed to find a file during a nushell operation.
     ///