Allow dynamic collection pagination

[nicodevs]

This PR allows paginating custom, dynamic collections. Fixes #273, although it does not use the config approach suggested there.

Issue

To paginate a collection, Jigsaw requires a source Blade template with a pagination key in its YAML front matter pointing at the target collection.

This breaks for dynamic collections, those generated at build time, like posts grouped by tag. You can build them in afterCollections, but each still needs a source file (source/tags/php.blade.php, source/tags/laravel.blade.php, source/tags/javascript.blade.php, etc.) created before you even know which tags exist.

Solution

With this PR, you can register paginated collections on the fly thanks to paginateCollection. The typical use case is tag/category pages. Let's say you want paginated pages of all your posts with certain tag ("PHP", "JavaScript", etc). In bootstrap.php:

$events->afterCollections(function (Jigsaw $jigsaw) {
    $posts = $jigsaw->getCollection('posts');

    $posts->flatMap(fn ($post) => $post->tags ?? [])
        ->unique()
        ->each(function ($tag) use ($jigsaw, $posts) {
            $slug = str($tag)->slug();
            
            $jigsaw->setConfig("tag_{$slug}", $posts->filter(fn ($p) => in_array($tag, $p->tags ?? [])));
            
            
            $jigsaw->paginateCollection(
                path: "tags/{$slug}",
                collection: "tag_{$slug}",
                template: '_layouts.tag',
                perPage: 5,
                variables: ['tag' => $tag],
            );
        });
});

That generates build/tags/php/index.html, build/tags/php/2/index.html, etc. Same pagination object ($pagination->items, $pagination->next, etc.) as the file-based approach, just driven from code instead of a source file.

What changed

• New Jigsaw::paginateCollection() API: registers a paginated page definition (path, collection, template, perPage, optional variables) for deferred rendering after collections are built
• PaginatedPageHandler::handleDefinition(): new method that renders paginated pages from a template view name instead of a source file
• CollectionPaginator::paginate(): refactored to accept relativePath and filename as separate strings instead of a $file object, decoupling it from the filesystem
• OutputFile: InputFile is now nullable to support pages that have no backing source file
• SiteBuilder::generatePaginatedPages(): new private method that processes pending page definitions and merges them into the build output
• ViewRenderer::renderView(): new method to render a view by name (using view()->make()) rather than by file path

How to test

1. In bootstrap.php, hook into afterCollections. Use teh snippet presented above.
2. Create a _layouts/tag.blade.php that uses $pagination.
3. Run ./vendor/bin/jigsaw build and verify build/tags/php/index.html and paginated sub-pages are generated correctly.
4. Run ./vendor/bin/phpunit tests/DynamicCollectionPaginationTest.php, all 5 tests should pass.