lang: ast: fix lambdas starting with var declaration not being parsed correctly

IVY_KW_VAR is not treated as an expression start token (as variable declarations are not
strictly expressions). so the lambda parser did not create a block parser context
when it encountered this keyword.

lang/ast/lambda.c

@@ -173,6 +173,9 @@ struct ast_node_type lambda_node_ops = {
 		SYM_PARSER(PIPE, parse_pipe),
 		SYM_PARSER(COLON, parse_colon),
 	},
+	.n_keyword_parsers = {
+		KW_PARSER(VAR, parse_expr_begin),
+	},
 	.n_expr_parser = {
 		.expr_begin = parse_expr_begin,
 	},