Fix lessopen feature

RawOsString API changed in os_str_bytes 7.0.0 and bat no longer builds
with the lessopen feature because:

- ::from_string() is deprecated and says to use new().
  The documentation says that new(), like from_string() no longer
  needs to copy the string.

- ::assert_from_raw_vec() is no longer merely deprecated and now
  requires selecting the "conversions" feature of os_str_bytes.

Replaces PR#2938

src/lessopen.rs

@@ -112,7 +112,7 @@ impl LessOpenPreprocessor {
                 }
 
                 (
-                    RawOsString::from_string(lessopen_stdout),
+                    RawOsString::new(lessopen_stdout),
                     path_str.to_string(),
                     OpenedInputKind::OrdinaryFile(path.to_path_buf()),
                 )