Fix nushell#10591: encode returns error with utf-16le and utf-16be encodings (nushell#10591) (#12411)

# Description

This closes (nushell#10591)

The Command encode's help text says that utf-16le and utf-16be encodings
are not supported, however you could still use these encodings and they
didn't work properly, since they returned the bytes UTF-8 encoded:
```bash
"䆺ש" | encode utf-16
Length: 5 (0x5) bytes | printable whitespace ascii_other non_ascii
00000000: e4 86 ba d7 a9 ×××××
 ```
# User-Facing Changes

The Command encode's help text was updated and now when trying to encode with utf-16le and utf-16be returns an error:
![screenshot](https://github.com/nushell/nushell/assets/119532691/c346dc57-8b42-4dfc-93d5-638b0041d89f)

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-command/src/strings/encode_decode/encode.rs

@@ -37,6 +37,7 @@ impl Command for Encode {
 big5, euc-jp, euc-kr, gbk, iso-8859-1, cp1252, latin5
 
 Note that since the Encoding Standard doesn't specify encoders for utf-16le and utf-16be, these are not yet supported.
+More information can be found here: https://docs.rs/encoding_rs/latest/encoding_rs/#utf-16le-utf-16be-and-unicode-encoding-schemes
 
 For a more complete list of encodings, please refer to the encoding_rs
 documentation link at https://docs.rs/encoding_rs/latest/encoding_rs/#statics"#

crates/nu-command/src/strings/encode_decode/encoding.rs

@@ -50,6 +50,19 @@ pub fn encode(
     } else {
         parse_encoding(encoding_name.span, &encoding_name.item)
     }?;
+
+    // Since the Encoding Standard doesn't specify encoders for "UTF-16BE" and "UTF-16LE"
+    // Check if the encoding is one of them and return an error
+    if ["UTF-16BE", "UTF-16LE"].contains(&encoding.name()) {
+        return Err(ShellError::GenericError {
+            error: format!(r#"{} encoding is not supported"#, &encoding_name.item),
+            msg: "invalid encoding".into(),
+            span: Some(encoding_name.span),
+            help: Some("refer to https://docs.rs/encoding_rs/latest/encoding_rs/index.html#statics for a valid list of encodings".into()),
+            inner: vec![],
+        });
+    }
+
     let (result, _actual_encoding, replacements) = encoding.encode(s);
     // Because encoding_rs is a Web-facing crate, it defaults to replacing unknowns with HTML entities.
     // This behaviour can be enabled with -i. Otherwise, it becomes an error.
@@ -102,9 +115,7 @@ mod test {
     #[case::iso_8859_1("iso-8859-1", "Some ¼½¿ Data µ¶·¸¹º")]
     #[case::cp1252("cp1252", "Some ¼½¿ Data")]
     #[case::latin5("latin5", "Some ¼½¿ Data µ¶·¸¹º")]
-    // Tests for specific renditions of UTF-16 and UTF-8 labels
-    #[case::utf16("utf16", "")]
-    #[case::utf_hyphen_16("utf-16", "")]
+    // Tests for specific renditions of UTF-8 labels
     #[case::utf8("utf8", "")]
     #[case::utf_hyphen_8("utf-8", "")]
     fn smoke(#[case] encoding: String, #[case] expected: &str) {