Add support for I18next JSONv3

[syeopite]

I18next is another popular standard translation format. It's features includes:

• Plurals
• Interpolation
• Formatting
• Nesting
• Objects and arrays

So, I think it's a good candidate for next major backend Lens implements. Perhaps in 0.2?

[SamantazFox]

Note that i18next plurals support can be extracted from iv-org/invidious#2646.

Everything is ready for v3, and a few minor modifications (and tests) are required to support v1 and v2.
A complete CLDR database is required for the plurals of v4.