Add nkf gem for Ruby 3.4 compatibility

Ruby 3.4 removed `kconv` from the standard library. The `xcodeproj` gem
(used by CocoaPods) depends on `kconv`, which is now provided by the
`nkf` gem. Without it, `bundle exec pod install` fails with:

  LoadError - cannot load such file -- kconv

Author: cipolleschi

template/Gemfile

@@ -14,3 +14,4 @@ gem 'bigdecimal'
 gem 'logger'
 gem 'benchmark'
 gem 'mutex_m'
+gem 'nkf'