Introduce FileNode to handle file-level changes

[TomasVotruba]

At the moment, there is no way to handle file-level operations, like adding declare(...) to a file. We hack around this using beforeTraverse(), which is internal part of php-parser and should be avoided.

This PR adds a FileNode to fix that. It allows to make changes directly in refactor() method as it should be 👍

final class DeclareStrictRector extends AbstractRector
{
    public function getNodeTypes(): array
    {
        return [FileNode::class];
    }

   /**
     * @param FileNode $node
     */
    public function refactor(Node $node): ?FileNode
    {
        if ($this->declareStrictTypeFinder->hasDeclareStrictTypes($node)) {
            return null;
        }

        $declaresStrictType = $this->nodeFactory->createDeclaresStrictType();
        $node->stmts = array_merge([$declaresStrictType, new Nop()], $node->stmts);

        return $node;
    }
}

It also opens-up possibility to have a single place to manage use statements (now handled by couple meta services). Next step will be marking beforeTraverse() and afterTraverse() final in AbstractRector, to keep BC promise only for RectorInterface (as it should be from the start).

This PR keeps BC as much as possible and adds reporting about deprecation to in custom rules.

---

Use of FileWithoutNamespace::class node: https://github.com/search?q=%22FileWithoutNamespace%3A%3Aclass%22+NOT+is%3Afork&type=code

Todo

• add BC layer to make FileWithoutNamespace still work
• add warning about it's use in getNodeTypes() and explain upgrade path

Related package PRs

• use FileNode in DeclareStrictTypesTestsRector rector-phpunit#599
• Make use of FileNode rector-doctrine#474
• Make use of FileNode - remove beforeTraverse() from nullsafe method call; counter is not relevant as output is already ternary nesting rector-downgrade-php#352

[TomasVotruba]

Let's ship it 👍