(feat) Alt Text Localization

[LucaArgentieri]

Description

This PR adds full support for localized asset fields in the media plugin, following Sanity's recommended localization scheme. Now, if a locale array is provided in the plugin config, all localizable fields (title, altText, description, creditLine) are shown in language-specific tabs, allowing editors to enter translations for each language. The implementation is compatible with objects containing extra keys (e.g. isDefault), but only id and title are used internally.

What to review

• The new tabbed UI for localized fields in the asset edit dialog.
• The dynamic schema and form logic for handling both localized and non-localized setups.
• TypeScript compatibility with extended locale objects.
• Documentation updates, including usage, config, and fallback handling.

Testing

• Manual testing with various locales arrays (including with extra keys like isDefault).
• Verified that fields are grouped by language in tabs and fallback logic is left to the user (with a GROQ example in the README).
• No automated tests were added, but the changes are isolated to the plugin’s asset field handling and UI.

Closes #265

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[evertharmeling]

Nice work @LucaArgentieri! It would be very nice if this functionality could be merged.