chore: make it compatible with phpcodesniffer-composer-installer

[EHER]

By changing the type on composer.json we allow the Totara code standard to be required via composer:

composer require --dev totara/codesniffer=dev-master

To have it working I also submitted the repository to Packegist (I hope you don't mind).

The Packegist page can be found here: https://packagist.org/packages/totara/codesniffer

For testing purposes, I also registered my fork on Packegist so we can run:

composer require --dev eher/totara-code-sniffer=dev-master

and check it with:

vendor/bin/phpcs -i