Co-authored-by: rhyd42 <rhydian.roberts@chinasearch.co.uk>

* [RELEASE_8.15.0] Add split feature

* [Docs] Add CONTRIBUTING.md

* Add `GotenbergAssetRuntime` to avoid passing Builder in context

Passing builder in context is fragile, as it won't work if user
uses any of Twig layout features without passing the full context.

Starting with Twig 4.0, the `include` function won't pass the
full context, to encourage template isolation and composition.

This PR introduce a Twig Runtime, lowering the impact of this extension
on the global scope, and allowing to create a time-limited "scope",
during which the `gotenberg_asset` will work as expected.

* Use Path:: methods instead of Filesystem instance

`Filesystem->isAbsolutePath` does not check if the file exists and just make quick checks on the string.

So here the Path utility methods seem a better choice as they are platform-agnostic and allow safe operations like `::join`  handle `../../` in paths and handle full path canonicalization.

And the tests can be done on any strings, independently on the Fixtures files here.

I think this method is used before calling `new File()`, so maybe the whole thing could be replaced by some factory/provider returning an Asset or something like that?

* Make 'sensiolabs_gotenberg.asset.base_dir_formatter' private