Rewrite plugin in Sanipack with native inputs

[SimeonGriggs]

I have rewritten the plugin entirely, it now:

• Is built on Sanipack for consistency and predictability of development
• Resolves inputs using FormBuilder instead of recreating any studio affordances, by leveraging NRK's package
• Reinforces best practices from Sanity's Localization docs and the @sanity/language-filter plugin for creating field-level translations
• Is designed as a Custom Input to selectively add a superset of features to the above style of field structure – instead of a helper function to create fields
• Expects field names to be language codes (just as @sanity/language-filter does)
• Contains some nicer UI with button tooltips, and toast messages and feedback for a missing API key
• Handles combination language/country codes like en_US
• Warns against field-level translation for Portable Text as this is an anti-pattern which should be discouraged
• Has no more console errors 😄

[SimeonGriggs]

I left all the existing GitHub Actions in the repo so maybe that explains the test error above :/