feat(api): Add Arg::get_value_terminator()

Expose get_value_terminator() for use by clap_complete.

Related-to: clap-rs#3022
Signed-off-by: David Aguilar <davvid@gmail.com>

src/builder/arg.rs

@@ -3880,6 +3880,13 @@ impl Arg {
         })
     }
 
+    /// Get the value terminator for this argument. The value_terminator is a value
+    /// that terminates parsing of multi-valued arguments.
+    #[inline]
+    pub fn get_value_terminator(&self) -> Option<&Str> {
+        self.terminator.as_ref()
+    }
+
     /// Get the environment variable name specified for this argument, if any
     ///
     /// # Examples