feat: Added x-default hreflang tag for HeadHrefLangs component

[Y-developer]

Describe your changes

I noticed that the HeadHrefLangs component in the astro-i18next package does not include the hreflang='x-default' tag. I added the hreflang='x-default' tag to the HeadHrefLangs component.

As a result of adding the hreflang='x-default' tag, I ran an audit using SEO Minion's Hreflang Checker before and after. The audit showed a difference in results.

Fixes #158

Checklist

• I have performed a self-review of my code
• I have added thorough tests
• I have updated the docs

[florian-lefebvre]

Looks like there's a merge conflict there

[lwhiteley]

had the same issue but solved it in a different way to keep the render data driven

---
const supportedLanguages = i18next.languages || [];
const currentUrl = Astro.url.href;
const defaultLanguage = AstroI18next.config.defaultLocale;

const languageAlternatives = [
    { hrefLang: 'x-default', language: defaultLanguage }, 
    ...supportedLanguages.map((language) => ({ hrefLang: language, language }))
];

---

{
  languageAlternatives.map(({hrefLang, language}) => (
    <link
      rel="alternate"
      hreflang={hrefLang}
      href={localizeUrl(currentUrl, language)}
    />
  ))
}