Add support for DuckDB functions named arguments with assignment operator (#1195)

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

src/ast/mod.rs

@@ -4584,6 +4584,8 @@ pub enum FunctionArgOperator {
     Equals,
     /// function(arg1 => value1)
     RightArrow,
+    /// function(arg1 := value1)
+    Assignment,
 }
 
 impl fmt::Display for FunctionArgOperator {
@@ -4591,6 +4593,7 @@ impl fmt::Display for FunctionArgOperator {
         match self {
             FunctionArgOperator::Equals => f.write_str("="),
             FunctionArgOperator::RightArrow => f.write_str("=>"),
+            FunctionArgOperator::Assignment => f.write_str(":="),
         }
     }
 }