Specify the standard smart quotes for Arabic

The alternative is the default for RTL languages.

crates/typst-library/src/text/smartquote.rs

@@ -264,6 +264,7 @@ impl<'s> SmartQuotes<'s> {
             "he" => ("’", "’", "”", "”"),
             "hr" => ("‘", "’", "„", "”"),
             "bg" => ("’", "’", "„", "“"),
+            "ar" if !alternative => ("’", "‘", "«", "»"),
             _ if lang.dir() == Dir::RTL => ("’", "‘", "”", "“"),
             _ => default,
         };