[FEATURE] Compatibility with PHP 7.3+ and Composer 2.0

[eliashaeussler]

This PR includes:

• Compatibility with PHP 7.3, 7.4 & 8.0 as well as Composer 2.0
• Replacement of var_export by serialize() for the generated BundleDescriber class to avoid circular references
• Support for GitHub actions as possible replacement for Travis CI

⚠️ Since extensive adaptions were made, this PR comes as breaking change for the current code base.

Version matrix

	PHP 8.0	PHP 7.4	PHP 7.3	PHP 7.2	PHP 7.1
Composer 1.x	0.5.x	0.5.x	0.5.x	0.1.0 - 0.5.x	0.1.0 - 0.5.x
Composer 2.x	0.5.x	0.5.x	0.5.x	0.5.x	0.5.x

The support for various versions is possible because ocramius/package-versions is now replaced by composer/package-versions-deprecated which does exactly the same besides the fact that it supports a wider range of platform versions.

However, we should keep in mind that the packages' name describes the actual state of the package: it's deprecated (but still supported). I'd suggest to keep it for now and remove the dependency once Composer 2.x has became established. We can then focus on the class generation provided by Composer.

Serialization

Since var_export cannot handle circular references (which is the case for the currently used installed packages objects), an alternative needed to be found. It turned out that serialization might be a good choice for now as it store the whole object/array representation and is also able to handle circular references.

As a result, var_export has now been replaced by serialize(). The PropertyTrait and InstalledPackagesTrait have been adapted to resolve all serialized occurrences on first access.

GitHub actions

This project now supports GitHub actions next to Travis CI. It would be great if we can get rid of Travis CI if it turns out that GitHub actions fulfill all our requirements:

• Test all supported combinations of PHP & Composer versions
• Collect test results and send them to SonarCloud

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

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[dwenzel]

@eliashaeussler Thank you so much. This is a great pull request!

Would you please comment on the usage of TestApplicationTrait?