parser: support null constant

trunk_parser/src/ast.rs

@@ -272,6 +272,7 @@ pub enum Expression {
     AnonymousClass(Option<Identifier>, Vec<Identifier>, Block),
     Bool(bool),
     ArrayIndex(Box<Self>, Option<Box<Self>>),
+    Null,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize)]

trunk_parser/src/parser/mod.rs

@@ -739,6 +739,10 @@ impl Parser {
                 self.next();
                 e
             },
+            TokenKind::Null => {
+                self.next();
+                Expression::Null
+            },
             TokenKind::LeftParen => {
                 self.next();