Support for PHP8 Union Types

[cseufert]

This change makes all function & method parameters, return types, and class member variables parse types as a list (array), rather than single items.

[coveralls]

Coverage decreased (-0.1%) to 96.055% when pulling 5aed303 on moddengine:php8-union-types into 7c2bbbc on glayzzle:master.

[cseufert]

This should close #656

[cseufert]

I think #693 is more inline with what was proposed in ticket #656

[cseufert]

I would not merge this and the v2 version

[cseufert]

#693 is a cleaner implementation

[czosel]

Oh sorry! I’ll revert this one later today.

[czosel]

I just realized that GitHub shows this MR as merged because #693 actually contains the commits of this MR as well. The git history might not be very clean, but otherwise I think we're good 😉