tpl/collections: Fix append when appending a slice to a slice of slices

[bep]

Fixes #11004

[jmooring]

hugo v0.114.0-DEV-4d22ba2

This:

{{ $obj := slice (slice "a") }}
{{ $obj = $obj | append (slice "b") }}
{{ $obj = $obj | append (slice "c") }}

<pre>
{{ jsonify (dict "indent" "  ") $obj }}
</pre>

Still produces this:

[
  [
    "a"
  ],
  "b",
  [
    "c"
  ]
]

[bep]

Hmm...

[bep]

I'm going to take one more stab at this, but I need to start fresh.

[github-actions]

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