feat: add formFiller to declaratively fill forms in page objects - #49

Adds formFiller, a helper to fill forms in page objects from a data object.

• Each key maps to a form input: anything with a fill method (e.g. a Locator or a nested page object) or a filler function such as (checked) => checkbox("Accept").setChecked(checked)
• undefined values are skipped
• fields are filled in the order of the fields object
• formFiller can be nested because it returns a filler function

import { formFiller, roleLocators } from "@cronn/playwright-utils";
import type { Page } from "@playwright/test";

function createRegistrationForm(page: Page) {
  const { button, textbox } = roleLocators(page);

  return {    
    fill: formFiller({
      name: textbox("Name"),
      email: textbox("Email"),
    }),
    submitButton: button("Register"),
  };
}