Mark types as "safe"

[lstrojny]

Today when a value object is inherently safe to output it must extend {{Twig\Markup}} and therefore introduce a hard dependency to Twig. It would be appreciated if one could reverse that relationship and mark a type as safe. Something like:

$env = new \Twig\Environment(…, ['object_safelist' => ['js' => [ObjectThatIsSafeForJs::class], 'html' => [Object1ThatIsSafeForOutputInHtml::class, Object2ThatIsSafe::class]]]);

twig_escape_filter() would use that list to look up if the passed object is considered safe for the current escaping and output the result of __toString() as is.

[fabpot]

see #3025 for a possible implementation.