[6.x] Keep augmentable tag results as objects inside Antlers interpolations

[duncanmcclean]

This pull request fixes an issue where assigning the asset tag's result to a variable, like {{ img = { asset url="/img/logo.png" } }}, gave you the URL string rather than the asset, so {{ img.alt }}, {{ img.width }} and friends were empty.

This was happening because tag output inside an interpolation was reduced as if it were being rendered directly, and that path casts any object with __toString (like an Asset) to a string before the value is handed back.

This PR fixes it by leaving augmentable objects alone when they're returned from a tag inside an interpolation. The Antlers runtime already knows how to work with an Asset held in a variable, so {{ img }} still outputs the URL, while {{ img.alt }}, {{ img:width }} and looping over {{ img }} now work too.

Fixes #11257