Adding minimal PHP-Version to composer package

[legionth]

Should also work for >= 8 versions.

[legionth]

Need to add composer.lock, but don't have composer on my machine. Will update tonight (or another night) with my other laptop.

[jayay]

This should also give Dependabot a hint on which versions of PHP must be supported. I'm not asking for this to be added unless you're absolutely sure what it does when this package is being used as a dependency in another project, since the documentation is being imprecise about this. See https://getcomposer.org/doc/06-config.md#platform
I want to avoid other projects using this package to get their dependencies pulled as if they'd be running PHP 7.0, even though they're not.

[legionth]

TIL platform :D

[jayay]

I think php-cs-fixer is the problem here.
What do you think about removing php-cs-fixer from the package.json for now, since the GH Action doesn't execute it anyway?
If desired, we could setup php-cs-fixer from a PHAR file in a separate action or task that runs a current version of PHP. Different ways to install it globally are documented here: https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/master/doc/installation.rst
That could be a follow-up PR, though.