Fix #9; adds static "type" property to conform to i18next API

[karoun]

What changed?

• See title; a static type property is required, per i18next documentation
• Raises plugin error when namespace is missing, rather than a console.error() (which I suspect was accidental)
• Turns i18next into a dev-only dependency since only its types are used; it isn't needed at plugin runtime

Test plan

• Ensure npm run build completes successfully