tpl/collections: Unwrap any interface value in sort and where

tpl/collections: Unwrap any interface value in sort and where

Hugo `0.55.0` introduced some new interface types for `Page` etc.

This worked great in general, but there were cases where this would fail in `where` and `sort`.

One such example would be sorting by `MenuItem.Page.Date` where `Page` on `MenuItem` was a small subset of the bigger `page.Page` interface.

This commit fixes that by unwrapping such interface values.

Fixes #5989