The type handling in these was improved in Hugo 0.49, but this also meant that it was no longer possible to start out with a string slice and later append `Page` etc. to it.
This commit makes sure that the old behaviour is now possible again by falling back to a `[]interface{}` as a last resort.
Fixes#5361
This commit consolidates the reflective collections handling in `.Scratch` vs the `tpl` package so they use the same code paths.
This commit also adds support for a corner case where a typed slice is appended to a nil or empty `[]interface{}`.
Fixes#5275
