helpers: Add option to transliterate content paths

[jmooring]

Closes #9134

[jmooring]

Supersedes #9135, rebasing against current master.

I am not expecting this to be merged right away. From comment on previous PR:

I'm in the process (as in: as we speak) of evaluating this whole path/URL/thing in Hugo, so I'm not merging this right right now. I agree that we probably need something like this, but it needs to wait.

[bep]

OK, arguments from your issue re our current solution:

1. Has no effect on non-composite characters (e.g., ß, ł, Ł)
2. Is not language aware (e.g., for German, ä should become ae)

---

I haven't looked at 1. but 2. The library in question does not look very complete re. the languages it supports. A random sample testing my name in my language failed (I expected Bjoern Erik Pedersen). I understand that this is fixable, but I fail to understand the scope of that work.

package main

import (
	"fmt"

	"github.com/alexsergivan/transliterator"
)

func main() {
	trans := transliterator.NewTransliterator(nil)
	fmt.Println(trans.Transliterate("Bjørn Erik Pedersen", "nn"))
}

[jmooring]

https://github.com/alexsergivan/transliterator/tree/master/languages

Supports nb, but not nn or no... so yes, language support is limited, with most falling back to default transliteration.

Another thing to consider with a language-specific implementation is whether to (a) base transliteration on the default content language (that's what this PR does), or (b) base transliteration on site language.

[istr]

For the extended version, it might be an option to use https://github.com/djimenez/iconv-go instead. This binds to the libiconv or glibc iconv functions (whatever your system provides) using cgo. As this could be considered an "extended" feature, it would make sense to provide it with full language support, but only in the extended version of hugo.