Version 2.1.0-beta.1

First beta release of Pico's official PicoDeprecated plugin as of Pico v2.1. This is a pre-release!

* [New] Add support for the latest API v3 changes
* [New] Support disabled Twig autoescape prior to API v3
* [New] Re-introduce `theme_url` config variable
* [New] Re-introduce `prev_page`, `base_dir` and `theme_dir` Twig variables
* [New] Support loading additional plugins using API v1 `onPluginsLoaded` event
* [New] Re-introduce Pico v0.9 config constant `CACHE_DIR`
* [New] Add release & build system to test the plugin using PHP_CodeSniffer and
        to automatically create pre-built release packages
* [Changed] Split the plugin's functionality into multiple compatibility
            plugins (two for each API version, for plugins and themes resp.)
            and load the necessary compatibility plugins on demand only; also
            allow 3rd-party plugins to load their own compatibility plugins