feat(router): introduce server support for form actions

[brandonroberts]

PR Checklist

Closes #935

What is the new behavior?

In .server.ts files, an action function can be defined to be used with forms to progressively enhance forms and build the foundation for handling form actions on the server.

// newsletter.server.ts
import {
  type PageServerAction,
  redirect,
  json,
  fail,
} from '@analogjs/router/server/actions';
import { readFormData } from 'h3';

export function load() {
  return {
    loaded: true,
  };
}

export async function action({ event }: PageServerAction) {
  const body = await readFormData(event);
  const email = body.get('email') as string;

  if (!email) {
    return fail(422, { email: 'Email is required' });
  }

  if (email.length < 10) {
    return redirect('/');
  }

  console.log({ email: body.get('email') });

  return json({ type: 'success' });
}

A follow-up PR will land to add a form directive to bridge the form to the form server action.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[optional] What gif best describes this PR or how it makes you feel?

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[jdgamble555]

whahoo!