save --append: create file if it doesn't exist (#4156)

* have save --append create file if not exists

Currently, doing:

echo a | save --raw --append file.txt

will fail if file.txt does not exist. This PR changes that

* test that `save --append` will create new file

crates/nu-engine/src/filesystem/filesystem_shell.rs

@@ -884,7 +884,7 @@ impl Shell for FilesystemShell {
     ) -> Result<OutputStream, ShellError> {
         let mut options = OpenOptions::new();
         if append {
-            options.append(true)
+            options.append(true).create(true)
         } else {
             options.write(true).create(true).truncate(true)
         };