PHP 8.1 Compatibility

[SychO9]

Hello! thanks for the package!

This PR updates the code to be able to support newer versions of PHP, as of right now while the package works, there are deprecation warnings in PHP 8.1

Updated code is:

• Tests to use phpunit 9.0

• Source code to use correct return types of implemented interfaces, and use __serialize() and __unserialize() as serialize() and unserialize() are deprecated.

• Should I remove the GitHub actions workflow file? I see there is Travis CI used but I left it just in case.

• I moved the source files into a src folder as I was having trouble running tests.

This PR alone doesn't work, it requires axy/errors to also be updated (axypro/errors#4) and a major version released as it completely renames a class.

[vasa-c]

@SychO9 hello! thanx! But why you changed version of axy/codecs-base64vlq to 2.0?

[SychO9]

Thank you for looking into this!

I forgot to push another PR to codecs, the package also uses axy/errors and since axy/sourcemap requires codecs they would clash so codecs would also need a new version that uses axy/errors 3.0 (https://github.com/axypro/codecs-base64vlq/blob/master/composer.json#L22)

[SychO9]

Opened the PR: axypro/codecs-base64vlq#1