fix(wasm): fix alias map size computation

This fixes a crash where parsing with certain languages can lead to a crash due to how the alias map was allocated and laid out in wasm memory

test/fixtures/test_grammars/external_tokens/scanner.c

@@ -30,7 +30,7 @@ void tree_sitter_external_tokens_external_scanner_destroy(void *payload) {
 unsigned tree_sitter_external_tokens_external_scanner_serialize(
   void *payload,
   char *buffer
-) { return true; }
+) { return 0; }
 
 void tree_sitter_external_tokens_external_scanner_deserialize(
   void *payload,