Add new rule identical-placeholders

[dhaldar-godaddy]

• This PR adds a new rule i18n-json\identical-placeholders to help validate the placeholders / variables used in the translations.
• A translator who is not very much familiar with the ICU message syntax, may end up changing (translating) the variables themselves, and breaking the string. Or there may be instances where the string is simply too complex and prone for mistakes.
• This new rule compares any new translations with a reference file ensuring the variables remain the same.