tpl/collections: Add group template func

This extends the page grouping in Hugo with a template function that allows for ad-hoc grouping.

A made-up example:

```
{{ $cool := where .Site.RegularPages "Params.cool" true | group "cool" }}
{{ $blue := where .Site.RegularPages "Params.blue" true | group "blue" }}
{{ $paginator := .Paginate (slice $cool $blue) }}
```

Closes #4865

tpl/collections/init.go

@@ -138,6 +138,11 @@ func init() {
 			[][2]string{},
 		)
 
+		ns.AddMethodMapping(ctx.Group,
+			[]string{"group"},
+			[][2]string{},
+		)
+
 		ns.AddMethodMapping(ctx.Seq,
 			[]string{"seq"},
 			[][2]string{