Fix missing constants in ScriptEvents in composer v2

[kranack]

• Composer v2 no longer uses constants in getSubscribedEvents method

This fix the error reported in #26 by @budziam

[staabm]

ping @barryvdh

[Tofandel]

I see you changed the package name on both master and the PR branch, but it doesn't seem published on packagist

Could you publish it and reset the PR branch to the previous commit?

[kranack]

@Tofandel I only changed the package name on my branch because I needed to use the package with composer v2. I could publish it on packagist but there's already some existing forks that already works with composer v2 (https://github.com/avto-dev/composer-cleanup-plugin as an example)

[barryvdh]

Thanks!