lang: fix underscopre not being treated as an expression start token

lang/ast/node.c

@@ -100,6 +100,7 @@ enum token_expr_type get_token_expr_type(struct ivy_token *tok)
 		case IVY_SYM_LEFT_BRACKET:
 		case IVY_SYM_LEFT_BRACE:
 		case IVY_SYM_CARET:
+		case IVY_SYM_UNDERSCORE:
 			return TOK_EXPR_BEGIN;
 		case IVY_SYM_COMMA:
 			return TOK_EXPR_ANY;