This commit adds support for return values in partials.
This means that you can now do this and similar:
{{ $v := add . 42 }}
{{ return $v }}
Partials without a `return` statement will be rendered as before.
This works for both `partial` and `partialCached`.
Fixes#5783
This means that you can do something ala:
```html
{{ if ge .Hugo.Version "0.36" }}Reasonable new Hugo version!{{ end }}
```
The intented use is feature toggling, but please note that it will take some time and Hugo versions until this can be trusted. It does not work in older Hugo versions.
Fixes#4443
