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_kotlin.cpp

@@ -156,7 +156,7 @@ class KotlinGenerator : public BaseGenerator {
     EnsureDirExists(dirs);
     const std::string filename =
         dirs + namer_.File(defname, /*skips=*/SkipFile::Suffix);
-    return SaveFile(filename.c_str(), code, false);
+    return parser_.opts.file_saver->SaveFile(filename.c_str(), code, false);
   }
 
   static bool IsEnum(const Type& type) {