tpl/urls: Add urls.PathUnescape function

[jmooring]

This would call Go's url.PathUnescape function.

Reference: https://discourse.gohugo.io/t/converting-a-linktitle-or-a-title-to-the-default-slug/56279

[jmooring]

For symmetry/round-trip I think it makes sense to add ursl.PathEscape as well.

[github-actions]

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