Add an appenln method to output_stream_t

This is an alternative to the very common pattern of

```rust
streams.err.append(output);
streams.err.append1('\n');
```

Which has negative performance implications, see https://github.com/fish-shell/fish-shell/pull/9229

It takes `Into<WString>` to hopefully avoid allocating anew when the argument is
a WString with leftover capacity

fish-rust/src/builtins/shared.rs

@@ -114,6 +114,12 @@ pub fn append<Str: AsRef<wstr>>(&mut self, s: Str) -> bool {
         self.ffi().append(&s.as_ref().into_cpp())
     }
 
+    /// Append a &wstr or WString with a newline
+    pub fn appendln(&mut self, s: impl Into<WString>) -> bool {
+        let s = s.into() + L!("\n");
+        self.ffi().append(&s.into_cpp())
+    }
+
     /// Append a char.
     pub fn append1(&mut self, c: char) -> bool {
         self.append(wstr::from_char_slice(&[c]))