Reinstate -a short form of save --append (#6575)

Present before engine-q merge (e.g.
265ee1281d) but not included when
--append was re-introduced at
https://github.com/nushell/nushell/pull/4744.

crates/nu-command/src/filesystem/save.rs

@@ -36,7 +36,7 @@ impl Command for Save {
         Signature::build("save")
             .required("filename", SyntaxShape::Filepath, "the filename to use")
             .switch("raw", "save file as raw binary", Some('r'))
-            .switch("append", "append input to the end of the file", None)
+            .switch("append", "append input to the end of the file", Some('a'))
             .category(Category::FileSystem)
     }