lang: ast: replace var keyword with global

local variables are now created automatically when they are first assigned to.

the global keyword can be used to declare that a name refers to a global variable instead,
at which point, assigning to the name results in the assignment referencing the global variable
instead of allocating a new global variable.

lang/ast/lambda.c

@@ -173,9 +173,6 @@ 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,
 	},