Improve contributor experience

[mk-mxp]

Can we add any CI job to ensure this formating is kept in next Pull Requests ?

Originally posted by @homersimpsons in #619 (comment)

• Ask exercism officials to add format checking to configlet workflow using a configuration switch
• Integrate configlet fmt into composer scripts (for contributors)
• Add a composer ci or such to run all tests with one command
• Add fetching configlet to composer install and composer update without loading it in CI
• Drop PHP 7.4 from composer.json

I think, this would greatly improve the contributor experience.