Make safe_join Twig filter return a Markup object

Problem/Motivation

If you have some safe strings and you want to safe_join them in a template and pass that as a @placeholder for translation, they get escaped even though they are safe. safe_join actually breaks the Markup object wrapper.

Why this should be an RC target

Without this patch any Markup instance passed to the |safe_join() Twig filter will be unintentionally escaped.

Disruptions: None, the intended result of this is to concatenate strings safely and return them joined by a separator and each item escaped if an item is not already a Markup instance.

Proposed resolution

Make the safe_join filter return a Markup object.

Remaining tasks

User interface changes

Less things getting accidentally escaped.

API changes

safe_join will return a TwigMarkup/Markup object rather than a string.

Data model changes

None

Why this should be an RC target

Expectations of how safe_join works with MarkupInterface will be way off and this is not disruptive.