add phpstan/psalm class-string types (phpdocs)

[staabm]

would you accept a PR which adds a class-string return type, where appropriate?

this would ease building new tools on top of php-parser, as we could typecheck things on top of it.

recent use-case composer-unused/composer-unused#111

[nikic]

At least as used in the PR referenced above, certainly not. Use of class-string is inappropriate in this context, as you are confusing classes in the code using PHP-Parser, and classes in the code parsed by PHP-Parser.