[Data Liberation] Filesystem entity reader

[adamziel]

A part of #1894

Adds a new API for loading content from a WP_Filesystem instance:

• WP_Filesystem_To_Post_Tree for traversing a directory tree and mapping the structure a hierarchical WordPress post/meta entity stream
• WP_Filesystem_Entity_Reader for converting the raw file content into WordPress blocks

To convert a set of zipped files into WordPress entities:

// Any Filesystem instance works here. Could be WP_Local_Filesystem,
// WP_Git_Filesystem, or anything else. Let's read from a zip file here:
$fs = new WP_Zip_Filesystem(
    WP_File_Reader::create('./docs.zip')
);

$reader = new WP_Filesystem_Entity_Reader( $fs );

foreach($reader as $entity) {
    var_dump($entity);
}

Testing

The code isn't used anywhere yet – just rely on the CI checks.