Add PHP 8.0 compatibility, fix iterating JSArray

[mdio]

Due to the usage of each() in JSArray::valid() the internal array pointer would be advanced every time valid was called.
This resulted in every odd element to be skipped (e.g. 1st, 3rd, 5th...) when iterating.

Fixes #56

[vierbergenlars]

Can you please rebase on master?

Then the tests will be run by GH actions.

[mdio]

After installing dependencies this fails with denied: installation not allowed to Write organization package while trying to publish a Docker container to the GitHub registry to run the tests in.
The ones for 7.3 and 7.4 already exist so it works there.
I have no how to fix this or how this is even supposed to work.
Maybe it's not possible to add new test targets in a PR from a fork?
It surely works on our fork with that config.

[vierbergenlars]

Yes, actions that are triggered by a PR from ourside an organization (or author) run with a github token that only has public read access (obviously, or you would be able to write to my container namespace)

I'll try to look into how to run some builds without needing to push a docker image to a registry.

[mdio]

Thank you for looking into that. I tried to search for it but I couldn't find a solution for it.

A temp fix I could think of is that you add the PHP 8.0 and 8.1 entries to the tests.yml in your repo.
That would of course mean that you get failing tests for that commit because PHPUnit 4 does not work with PHP 8.0.

Also JFYI: I will be on vacation for 2 weeks from tomorrow on so I won't be able to work on my fork in the meantime or answer here.