tpl/collections: Add Pages support to Intersect and Union

tpl/collections: Add Pages support to Intersect and Union

This enables `AND` (`intersect`)  and `OR` (`union`)  filters when combined with `where`.

Example:

```go
{{ $pages := where .Site.RegularPages "Type" "not in" (slice "page" "about") }}
{{ $pages := $pages | union (where .Site.RegularPages "Params.pinned" true) }}
{{ $pages := $pages | intersect (where .Site.RegularPages "Params.images" "!=" nil) }}
```

The above fetches regular pages not of `page` or `about` type unless they are pinned. And finally, we exclude all pages with no `images` set in Page params.

Fixes #3174