Allow pluralization customization via constructor options (closes #464)

[Raiondesu]

Closes #464.

Summary

Allows for a more civilized declarative style of defining pluralization rules via constructor options. This is needed to allow pluralization customization in environments where direct VueI18n prototype modification is not available (like nuxt, for example).

This PR changes the way custom pluralization would work in a backwards-compatible manner, which means - usages that utilize custom pluralization as defined in #451 will still work perfectly.

---

API changes

Instead of direct prototype modification it is now recommended to pass an optional pluralizationRules object into VueI18n constructor options.

Very simplified example using rules for Slavic languages (Russian, Ukrainian, etc.):

new VueI18n({
  pluralizationRules: {
    // Language to use the rule for, 'ru', in this case
    /**
     * @param choice {number} a choice index given by the input to $tc: `$tc('path.to.rule', choiceIndex)`
     * @param choicesLength {number} an overall amount of available choices
     * @returns a final choice index to select plural word by
    **/
    'ru': function (choice, choicesLength) {
      // this === VueI18n instance, so the locale property also exists here

      if (choice === 0) {
        return 0;
      }

      const teen = choice > 10 && choice < 20;
      const endsWithOne = choice % 10 === 1;

      if (choicesLength < 4) {
        return (!teen && endsWithOne) ? 1 : 2;
      }
      if (!teen && endsWithOne) {
        return 1;
      }
      if (!teen && choice % 10 >= 2 && choice % 10 <= 4) {
        return 2;
      }

      return (choicesLength < 4) ? 2 : 3;
    }
  }
});

This would effectively give this:

const messages = {
  ru: {
    car: '0 машин | 1 машина | {n} машины | {n} машин',
    banana: 'нет бананов | 1 банан | {n} банана | {n} бананов'
  }
}

Where the format is 0 things | 1 thing | few things | multiple things.

Your template still needs to use $tc(), not $t():

<p>{{ $tc('car', 1) }}</p>
<p>{{ $tc('car', 2) }}</p>
<p>{{ $tc('car', 4) }}</p>
<p>{{ $tc('car', 12) }}</p>
<p>{{ $tc('car', 21) }}</p>

<p>{{ $tc('banana', 0) }}</p>
<p>{{ $tc('banana', 4) }}</p>
<p>{{ $tc('banana', 11) }}</p>
<p>{{ $tc('banana', 31) }}</p>

Which results in:

<p>1 машина</p>
<p>2 машины</p>
<p>4 машины</p>
<p>12 машин</p>
<p>21 машина</p>

<p>нет бананов</p>
<p>4 банана</p>
<p>11 бананов</p>
<p>31 банан</p>

Default pluralization

If your current locale is not found in a pluralization map, the default rule of the english langugage will be used.

---

This is achieved by checking for current locale in a pluralizationRules object. If the locale is not found, VueI18n will proceed with the old implementation.

Prototype modification (as described in #451) will simply work around this concept without causing any disturbances, as demonstrated in this test.

P.S.

This is a possible minor update, essentially setting the version of vue-i18n to 8.5.0.

[codecov-io]

Codecov Report

Merging #482 into dev will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #482      +/-   ##
==========================================
+ Coverage   96.64%   96.66%   +0.02%     
==========================================
  Files           9        9              
  Lines         655      660       +5     
==========================================
+ Hits          633      638       +5     
  Misses         22       22

Impacted Files	Coverage Δ
src/index.js	99.63% <100%> (ø)	⬆️

---

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[codecov-io]

Codecov Report

Merging #482 into dev will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #482      +/-   ##
==========================================
+ Coverage   96.64%   96.66%   +0.02%     
==========================================
  Files           9        9              
  Lines         655      660       +5     
==========================================
+ Hits          633      638       +5     
  Misses         22       22

Impacted Files	Coverage Δ
src/index.js	99.63% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4a8c27...5abac75. Read the comment docs.

[kazupon]

Great work!
Thanks!

Could you check my review comment please?

[kazupon]

Where Intl type definition ?

[Raiondesu]

It's included in ambient TS types and represents the internationalization options for toLocaleString functions.
https://github.com/Microsoft/TypeScript/blob/eb2297df022f6b1f284aff96e93c06097bb01ea5/lib/lib.es5.d.ts#L4093

[kazupon]

Thanks!
However, In about typing change, I prefer that it's changed with another PR, because different from the this issue to be solved.

[Raiondesu]

Ok, I can see where you're coming from on this one, I'll revert it rn.