Default @posts to main query and accept query arg

[mmirus]

Thanks for this set of directives, @Log1x!

For your consideration...

This PR changes @posts to provide greater flexibility:

1. Now defaults to the main query instead of $query
2. Now can accept an optional argument so that you can pass a query to be used in

This would break backwards compatibility with previous @posts usage that defaulted to using $query.

Options for moving forward:

1. Accept breaking change and use semver / docs to communicate it appropriately and help users know what they need to update (@posts -> @posts($query))
2. Modify this PR so that if no query is passed to the directive and $query exists it defaults to that instead of to the main query

Updated docs:

@posts

@posts begins the post loop. It is wrapped in a have_posts() conditional and thus will return null if no posts are found.

@endposts is available to end your loop and have_posts() conditional as well as resetting your loop with wp_reset_postdata().

@posts defaults to the main query, but it accepts a WP_Query instance as an optional argument. If the argument is provided, @posts will use that query instead.

@posts
  <h2 class="entry-title">{!! get_the_title() !!}</h2>
  <div class="entry-content">
    @php the_content() @endphp
  </div>
@endposts

@posts($other_query)
  <h2 class="entry-title">{!! get_the_title() !!}</h2>
  <div class="entry-content">
    @php the_content() @endphp
  </div>
@endposts