Add Tajik (tg) language support for plural rules

[ValeryVerkhoturov]

Summary

Added support for the Tajik language (ISO 639-1 code: tg) to the plural rules system.

Changes

• Added tg to the list of languages in internal/plural/codegen/plurals.xml that follow the same pluralization rules as Persian (Farsi)
• Regenerated internal/plural/rule_gen.go and internal/plural/rule_gen_test.go using the codegen script
• All tests pass successfully

Pluralization Rules

Tajik follows the same CLDR plural rules as Persian (fa), source:

• One: when i = 0 or n = 1
• Other: for all other numbers

Testing

go test ./internal/pluralAll existing tests continue to pass, and the new TestAmAsBnDoiFaGuHiKnPcmTgZu function validates the Tajik plural rules.

[ValeryVerkhoturov]

Closes #389

[ValeryVerkhoturov]

@nicksnyder Make review please