Ability to configure defaults in createInertiaApp()

[pascalbaljet]

Definitely WIP, but I'm already opening this to gather feedback/input.

This PR introduces a new defaults key to the CreateInertiaAppProps interface, allowing you to define application-wide defaults.

For example, you can use it to change the default queryStringArrayFormat from brackets to indices, or to attach default headers to every request:

createInertiaApp({
  title: (title) => `${title} - ${appName}`,

  // ...resolve, setup, progress, etc...

  defaults: {
    visitOptions: (href, options) => {
      return {
        queryStringArrayFormat: 'indices',
        headers: {
          'X-Foo': 'bar',
        },
      }
    },
  },
})

I've chosen to nest visitOptions under defaults so we don't bloat CreateInertiaAppProps if we end up adding more of these callbacks later on.