The fix introduced in Hugo `0.49.1` had an unintended side-effect in the `Append` func used in both `append` and `.Scratch.Add`.
This commit fixes that by loosen/fixing the type checking so concrete types can be appended to interface slices.
Fixes #5303
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
