Add ability to register private blocks, sets and presets

[robdekort]

This PR by @marcorieser and me is a refactor of the commands addon that adds the ability to register custom private locations for blocks, sets and presets.

To register custom content:

1. Publish the config file.
2. Add paths for either blocks, sets or presets.
3. Make sure each block, set or preset contains a config.php using the following pattern.

<?php

return [
    'handle' => 'pricing',
    'name' => 'Pricing tiers & features',
    'description' => 'Create and list pricing tiers and feature tables.',
    'operations' => [
        // Adds the ability to rename a collection handle. Use `{{ handle }}` in your files.
        [
            'type' => 'rename'
        ],
        // Runs a custom CLI command.
        [
            'type' => 'run',
            'command' => 'composer require vendor/package',
            'processing_message' => 'Installing vendor/package',
            'success_message' => 'Package installed.'
        ],
        // Copy a file, optionallly add the option to skip the operation.
        [
            'type' => 'copy',
            'input' => 'source.file',
            'output' => 'destination.file',
            'skippable' => false // Default.
        ],
        // Update the page builder with a new block.
        [
            'type' => 'update_page_builder',
            'block' => [
                'name' => 'Title',
                'instructions' => 'Instructions',
                'icon' => 'icon',
                'handle' => 'handle',
            ]
        ],
        // Update Bard with a new set.
        [
            'type' => 'update_article_sets',
            'block' => [
                'name' => 'Title',
                'instructions' => 'Instructions',
                'icon' => 'icon',
                'handle' => 'handle',
            ]
        ],
        // Add permissions to role. Use `{{ handle }}` when working with a renamable collection.
        [
            'type' => 'update_role',
            'role' => 'role_handle',
            'permissions' => ['view {{ handle }} entries', 'edit {{ handle }} entries', 'create {{ handle }} entries', 'delete {{ handle }} entries', 'publish {{ handle }} entries', 'reorder {{ handle }} entries', 'edit other authors {{ handle }} entries', 'publish other authors {{ handle }} entries', 'delete other authors {{ handle }} entries']
        ],
        // Notify the user with a message in the CLI. The user has to hit enter to continue.
        [
            'type' => 'notify',
            'content' => "All done."
        ],
    ]
];