add linter and ensure coding standards

[eug-L]

Added phpcs with Drupal coding standard, with some tweaks:

• Excluded Drupal.Classes.ClassFileName.NoMatch which requires class name to match file name
• Excluded Drupal.Commenting as it is too restrictive on comment sentences (start with capital letter, full stops)
• Added rules from Squiz.Commenting to enforce @param and @return on function calls
• Added Squiz.PHP.DisallowMultipleAssignments.FoundInControlStructure to restrict assignment in if statements