refactor(js): introduced @genkit-ai/shared package that will contain shared type, plugin and action framework

[pavelgj]

@genkit-ai/shared is going to be runtime agnostic. It may largely replace @genkit-ai/core in the future except @genkit-ai/core may remain the node specific isntrumentation.

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)