Avoid GetText deprecation warning when using newer GetText

[JasonTrue]

Summary of changes

(Please note: this summary has been substantially edited to reflect my current understanding per @Sinc63's comments).

Newer versions of gettext are pickier about the Plural-Forms header structure than before, and will ignore the nplurals behavior unless there is 1) a rule for which plural form to support and 2) the rule is in the expected format.

To avoid deprecation warnings, the Plural form rules need to be corrected. Alternatively, custom implementations of the GetText.Plural behavior can be used, especially if custom behavior is required. See GetText.Plural module documentation

As far as I can tell, the rules used by Timex mirror the rules implemented by the transitive dependency gettext 0.22 now has on Expo, so I don't see an immediate need for a custom implementation of Gettext.Plural.

This PR also removes an incorrectly specified plural rule for Indonesian.

There may be some remaining issues with Russian and possibly languages with similar pluralization rules, like Croatian, Polish, and Serbian, as several unit tests for Russian still fail.

Checklist

• [x] New functions have typespecs, changed functions were updated
• [x] Same for documentation, including moduledocs
• [x] Tests were added or updated to cover changes
• [x] Commits were squashed into a single coherent commit
• [x] Notes added to CHANGELOG file which describe changes at a high-level

[JasonTrue]

Fixes #734.

[maennchen]

so I don't see an immediate need for a custom implementation of Gettext.Plural

That would only be needed if you implement pluralization yourself. Here this is not needed.

[travismorrison]

Hi--any thoughts on when this will get merged? we're mildly blocked on updating deps on one of our apps.

[KernelMadness]

Hi, also want to support this PR, looks like it needs only one review from @maennchen

[bitwalker]

I'll get this released ASAP

[warmwaffles]

@bitwalker any chance for a release with this fix included?

[bitwalker]

Done