Implement --file-names-only (#8788)

* flatc builds and seems to work, some of the extra targets are having linker errors

* fix build system

* pipeline failures

* un-rename files

* refactor to use unique_ptr

* typo

* rm make_unique, add comments

* fix cmake

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Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

src/idl_gen_json_schema.cpp

@@ -330,7 +330,7 @@ class JsonSchemaGenerator : public BaseGenerator {
 
   bool save() const {
     const auto file_path = GeneratedFileName(path_, file_name_, parser_.opts);
-    return SaveFile(file_path.c_str(), code_, false);
+    return parser_.opts.file_saver->SaveFile(file_path.c_str(), code_, false);
   }
 
   const std::string getJson() { return code_; }