session: opt for enabling directionality markers

Add an option for enabling and disabling Fluent's directionality
isolation markers in output. Disabled by default as these can render in
some terminals and applications.

Signed-off-by: David Wood <david.wood@huawei.com>

src/parse/session.rs

@@ -114,7 +114,7 @@ fn default_handler(
     let emitter = if hide_parse_errors {
         silent_emitter()
     } else {
-        let fallback_bundle = rustc_errors::fallback_fluent_bundle()
+        let fallback_bundle = rustc_errors::fallback_fluent_bundle(false)
             .expect("failed to load fallback fluent bundle");
         Box::new(EmitterWriter::stderr(
             color_cfg,