tpl/collections: Add collections.Append

Before this commit you would typically use `.Scratch.Add` to manually create slices in a loop.

With variable overwrite in Go 1.11, we can do better. This commit adds the `append` template func.

A made-up example:

```bash
{{ $p1 := index .Site.RegularPages 0 }}{{ $p2 := index .Site.RegularPages 1 }}
{{ $pages := slice }}
{{ if true }}
  {{ $pages = $pages | append $p2 $p1 }}
{{ end }}
```

Note that with 2 slices as arguments, the two examples below will give the same result:

```bash
{{ $s1 := slice "a" "b" | append (slice "c" "d") }}
{{ $s2 := slice "a" "b" | append "c" "d" }}
```

Both of the above will give `[]string{a, b, c, d}`.

This commit also improves the type handling in the `slice` template function. Now `slice "a" "b"` will give a `[]string` slice. The old behaviour was to return a `[]interface{}`.

Fixes #5190

tpl/resources/resources.go

@@ -88,22 +88,11 @@ func (ns *Namespace) Concat(targetPathIn interface{}, r interface{}) (resource.R
 	var rr resource.Resources
 
 	switch v := r.(type) {
-	// This is what we get from the slice func.
-	case []interface{}:
-		rr = make([]resource.Resource, len(v))
-		for i := 0; i < len(v); i++ {
-			rv, ok := v[i].(resource.Resource)
-			if !ok {
-				return nil, fmt.Errorf("cannot concat type %T", v[i])
-			}
-			rr[i] = rv
-		}
-	// This is what we get from .Resources.Match etc.
 	case resource.Resources:
 		rr = v
+	case resource.ResourcesConverter:
+		rr = v.ToResources()
 	default:
-		// We may support Page collections at one point, but we need to think about ...
-		// what to acutually concatenate.
 		return nil, fmt.Errorf("slice %T not supported in concat", r)
 	}