Fix plural issue with lang-region locale

[xu3u4]

To fix #256

For {language}-{region} format locale, the plural count key can't be generated correctly.
The locale from the result of pluralRule.resolvedOptions(); may only contain the language part, without the region.
Although the value of locale should be BCP 47 language tag, but node(only tested on v18.17) and some browsers don't support it yet. So I think we can also compare the language part of the locale.

On node v18.17

const pluralRules = new Intl.PluralRules('en-US');
const options = pluralRules1.resolvedOptions();

console.log(options);
// output: Object { locale: "en", type: "cardinal",....}

[xu3u4]

Hi @gilbsgilbs
May I have your review when you have time 🙏

[gilbsgilbs]

Thanks for the PR, I'll look into it ASAP. In the meantime, could you add a test for your fix?

[xu3u4]

Thank you for your suggestion 🙇‍♀️ , just added!

[xu3u4]

@gilbsgilbs
I saw your change that using startWith looks good!
Thank you very much.

[gilbsgilbs]

I'll make a release later this week, probably tonight or tomorrow. I want to do some chore before releasing (mainly update dependencies, drop EOLed node versions and officially support newer node versions).

[gilbsgilbs]

Just published it under the 0.9.1 tag on NPM. Thank you again for your contribution, that's appreciated.