tpl: Create functions to inspect the union file system

[jmooring]

Reference: https://discourse.gohugo.io/t/methods-to-traverse-the-virtual-filesystem/49058/

{{ ufs.ReadDir }}
{{ ufs.FileExists }}

Why not just modify os.ReadDir and os.FileExists?

1. We wouldn't have to worry about breaking existing sites
2. We're not inspecting the OS file system; we're inspecting the union file system

[bep]

{{ ufs.ReadDir }}

Maybe fs.ReadDir would mke more sense -- it's not likely we're going to invent another file system.

There are some potential suprises, though, which we need to think about before doing this. This is component dependant, but e.g. doing fs.ReadDir "content/posts" may return multiple files with the same name (with different languages). Also, the behaviour of these are very much tied to "what Hugo needs", which may change.

So we can add fs.FileExists, but we need to think about the ReadDir.

[cmahnke]

See also #11917