[TASK] Add (and use) a CSSListItem type

[JakeQZ]

This allows a single type to be used for the contents of a CSSList, instead of a long list of orred types, and helps with static analysis.

Various assertInstanceOf() tests are added to the test cases to confirm that the list items are of the type expected.

Some implements and exetends lists are now alphabetically sorted.

[JakeQZ]

Notes:

• I have marked this as blocked by [CLEANUP] Return null from DeclarationBlock::parse() on failure #1209, which is a pre-patch for this;
  • This PR includes that change, so if that PR is accepted, it should be possible to merge this PR without conflict;
• When a type is A|B, PHPStan is happy if either A or B has the method invoked,
  • but if A and B both implement C, and the type is C, it requires that C defines the method, or an additional type check is performed - hence the need for additional assertInstanceOf() checks in the tests;
• I think the new PHPStan error arises because it would now be possible to have CSSListItem|array as a native parameter type - but not in PHP 7.2 -
  • It would be nice if PHPStan would allow specification of a target PHP version separately from the version it runs on, so it would not generate errors that can't be fixed for the minimum target PHP version;
    • IIRC, this was not possible with Psalm, but maybe it is possible with PHPStan;
    • Running PHPStan on PHP 7.2 would almost certainly not permit the latest version.

[coveralls]

coverage: 52.048%. remained the same
when pulling 691cfb5 on task/csslistitem-type
into acfe85e on main.

[JakeQZ]

Apparently PHP<7.4 does not like implementing an interface that is already implemented. That was the other note I was going to add - that I kept that redundancy in for clarity and completeness. Seems we can't :/

[JakeQZ]

I've fixed the issues with PHP<7.4, and rebased. This can be reviewed now.