Feature: Switch to Composer autoloader

[cameronterry]

This PR includes:

• Introduced composer autoloader to the plugin.
• Beginning to rename parts of the code from "DarkMatter" to "DarkMatterPlugin".
• Added a dependency for the spatie/php-structure-discoverer, which is used to load classes within the Dark Matter Plugin namespace.
• Added strauss dependency to prefix vendor/ and relevant dependencies to prevent conflicts with other code within a WordPress installation.
• Added helper interfaces to be used with handling classes to be loaded and Commands for CLI.
• Refactored a number of classes into the new structure.
  • Moved the Database class to the new autoloader approach.
  • Moved the UI class to the new autoloader approach.
  • Moved CLI into the new autoloader approach.
    • Renamed list() methods to _list() to removed a PHPCS warning (that was suppressed).
    • Backward compatibility is maintained by making use of the comment, @subcommand list (https://make.wordpress.org/cli/handbook/guides/commands-cookbook/#docblock-tags).
  • Moved REST API classes into the new autoloader approach, and move it into a versioned /v1/ folder.
  • Moved Yoast and compatible "allow logins" into new classes within Compat.
  • Note: classes which are likely to introduced backwards compatibility uses and used in custom logic by installs were left as-is.
  • This will be handled in the future 3.0.0 major release, where it is expected to have backwards compatibility issues.