Fix C23 syntax errors

```
error C2059: syntax error: '}'
```

Author: soutaro

src/lexstate.c

@@ -92,7 +92,7 @@ static const char *RBS_TOKENTYPE_NAMES[] = {
   "tANNOTATION",      /* Annotation */
 };
 
-rbs_token_t NullToken = { .type = NullType, .range = {} };
+rbs_token_t NullToken = { .type = NullType, .range = {0} };
 rbs_position_t NullPosition = { -1, -1, -1, -1 };
 rbs_range_t NULL_RANGE = { { -1, -1, -1, -1 }, { -1, -1, -1, -1 } };
 

src/parser.c

@@ -3415,7 +3415,7 @@ rbs_parser_t *rbs_parser_new(rbs_string_t string, const rbs_encoding_t *encoding
     .vars = NULL,
     .last_comment = NULL,
 
-    .constant_pool = {},
+    .constant_pool = {0},
     .allocator = allocator,
     .error = NULL,
   };

src/util/rbs_constant_pool.c

@@ -96,7 +96,7 @@ rbs_constant_pool_resize(rbs_constant_pool_t *pool) {
 }
 
 // This storage is initialized by `Init_rbs_extension()` in `main.c`.
-static rbs_constant_pool_t RBS_GLOBAL_CONSTANT_POOL_STORAGE = {};
+static rbs_constant_pool_t RBS_GLOBAL_CONSTANT_POOL_STORAGE = {0};
 rbs_constant_pool_t *RBS_GLOBAL_CONSTANT_POOL = &RBS_GLOBAL_CONSTANT_POOL_STORAGE;
 
 /**