feat: add php 8.3 support and deprecate php 7

feat: improve readme (#35)

* feat: improve readme

* feat: improve readme

* feat: improve readme

Minor fixes

• Update Composer release date. Noticed by https://www.youtube.com/watch?v=v2IjMrpZog4&lc=UgyBUuMgOfZkEeNb06x4AaABAg
• Fix PhpUnit test suite name
• Set Codelyber#GREETING constant as private in order to encapsulate it avoiding calling it form test cases

Update PHP and project dependencies

• Improve Readme.md including step by step guides on how to start developing
• Upgrade to PHP 7.2
• Update dependencies:
  • "jakub-onderka/php-parallel-lint": "^0.9",
  • "jakub-onderka/php-parallel-lint": "^1.0",
  • "jakub-onderka/php-console-highlighter": "^0.3",
  • "squizlabs/php_codesniffer": "^2.7",
  • "phpunit/phpunit": "^5.7",
  • "symfony/var-dumper": "^3.2"
  • "squizlabs/php_codesniffer": "^3.3",
  • "phpunit/phpunit": "^7.2",
  • "symfony/var-dumper": "^4.1"