feat: enable "substring" as a UDF in addition to "substr"

Substrait uses the name "substring", and it already exists in DF SQL

The setup here is a bit weird; I'd have added substring as an alias for substr, but then we have here this "substring" version being created as udf already and exported through the export_functions, with slightly different args than substr
(even though in reality the underlying function for both is the same substr impl).

I think this PR should work, but if you have suggestions on how to make the situation here cleaner, I'd be happy to!

datafusion/functions/src/unicode/mod.rs

@@ -138,6 +138,7 @@ pub fn functions() -> Vec<Arc<ScalarUDF>> {
         strpos(),
         substr(),
         substr_index(),
+        substring(),
         translate(),
     ]
 }