Problem/Motivation
Now that we're well on our way to managing coding standards in a reproducible way: #2571965: [meta] Fix coding standards in core
...let's just add the tools to core's composer.json dev requirements.
That way we don't have to instruct people how to install them beyond saying "run composer install --dev"
Proposed resolution
Modify core/composer.json to add drupal/coder and squizlabs/php_codesniffer as a dev requirement.
Add an install script to composer.json which adds coder's code_sniffer as an installed_path.
Remaining tasks
Update documentation such as https://www.drupal.org/node/1419988 and https://www.drupal.org/node/1587138