Add _zero suffix lookup if count === 0

[kgominyuk]

Localizing in a specific way 0 count is a very common case.
There was a related issue.
Also unicode documentation says: "implementations are encouraged to provide the ability to have special plural messages for 0 in particular, so that more natural language can be used" https://cldr.unicode.org/index/cldr-spec/plural-rules

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided

[adrai]

Currently it's what the Intl plural resolves to...

I don't know if we should introduce a special zero handling...

What do you think @jamuhl?

[kgominyuk]

This approach is very common:
RubyOnRails - https://guides.rubyonrails.org/i18n.html#pluralization
Apple - https://developer.apple.com/documentation/xcode/localizing-strings-that-contain-plurals

[jamuhl]

For me ok...as it's a rather common use case. But I think the check should add && (pluralSuffix === 'one' || pluralSuffix === 'other') to avoid using it with v3 and using it on languages like chinese where the only used form is other

[kgominyuk]

I added this.pluralResolver.shouldUseIntlApi() into condition to avoid using it with v3.
Adding && (pluralSuffix === 'one' || pluralSuffix === 'other') won't prevent zero lookup for chinese.

Why you think that this is irrelevant for chinese?

The goal of this change is to provide the ability to use more natural language:
None of your friends are online. instead of You have 0 friends online.

This is also actual for chinese.

[jamuhl]

It's a guess that chinese as it has only one plural form does not have a thing like zero, one, other at all...but idk...I'm not capable of speaking any asian language

but you're right...the check won't work anyway...as the suffix is other

[kgominyuk]

Also want to mention that this pr isn't introduce requirement to add _zero entries for all localized keys.
If the count is zero, and a _zero entry is present, then it will be used.
If _zero entry isn't present, original plural suffix will be used.

[coveralls]

Coverage increased (+0.06%) to 90.941% when pulling d48a453 on kgominyuk:zero-suffix-support into 22a45fa on i18next:master.

[adrai]

needs to be documented somewhere

[adrai]

Thank you for your contribution, it's included in v21.6.0