refactor(js): move plugin definition to ai package

[pavelgj]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request refactors the location of core plugin interfaces and types to improve package structure. Specifically, the GenkitPluginV2 interface has been moved to the @genkit-ai/ai package, and the ResolvableAction type has been moved to @genkit-ai/core. The changes also include a cleanup of imports across several plugin packages to consistently use the import type syntax for type-only dependencies. I have no feedback to provide as there were no review comments.