WIP tpl: Add TemplateFuncsNamespaceRegistry

[bep]

As a first step to remove the hard ties between tplimpl and the different namespace packages.

The lang package is used as the first example use case.

See #3042

/cc @moorereason

[bep]

@moorereason I will wrap this up tomorrow sometime, but if you can look at the test cases left:

https://github.com/bep/hugo/blob/4f37573810762be608fb74e03d2c5d2b5cca17a3/tpl/tplimpl/template_funcs_test.go#L129

And ignore the obvious ones (the few I have not moved). Where do the rest belong.

• printf etc. new package fmt I assume?
• urlize ... in urls?
• htmlUnescape etc.?
• dateFormat

Also these:

https://github.com/bep/hugo/blob/b55f4713621dfd7de6a3ca13202252daa428907a/tpl/tplimpl/template_funcs.go#L96

[moorereason]

• htmlEscape and htmlUnescape are in transform.
• dateFormat is time.Format.
• A fmt namespace sounds appropriate. 👍
• urls.Urlize 👍
• For string and int, do we add a reflect namespace? Don't know why it would be necessary at this point, but we could have methods for many types ([u]int[8|16|32|64], float[32], ...).
• partial* to new templates namespace?

[bep]

@moorereason if you're up and about: The only thing left now is some badly named partials methods. Suggestions?

[bep]

Never mind; I name it partials.Include -- and then handle the cache variant in #3417

[github-actions]

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