Use latest slevomat/coding-standard

slevomat/coding-standard 7.0.0 renames some DeclareStrictTypes options and changes their meaning: previously newlines were counted, now just lines are counted. Where there was one newline between the opening tag and the declare statement, there's now no line (but still one newline).

Class structure

Add rule to check order of classes structure, i.e. use first, then constants, properties, abstract methods, constructor, other methods.

Sort imports

• Sort uses (thanks @ruudk)

PHP 8

Can now be used for PHP 8 projects too (#1, thanks @xificurk)