Add support for actions API

[BryceRussell]

Add support for the new experimental actions API from Astro

Things to consider:

• This is an experimental feature. For now, this should only be enabled if an author defines an entrypoint
• This only works with SSR, ATP needs to warn users that a project is SSG and not SSR or Hybrid

[BryceRussell]

Blocked until support for injecting actions is added: https://github.com/withastro/roadmap/blob/actions/proposals/0046-actions.md#integration-actions

Current recommendation is to export an action from the theme package for users to configure themselves

// package/actions.ts
import { defineAction, z } from 'astro:actions';

export const someActions = () => {
    like: defineAction({
        // ...
    })
}

// project/src/actions/index.ts
import { defineAction, z } from 'astro:actions';
import { someActions } from 'my-theme/actions';

export const server = {
  ...someActions()
};