feat: implement PSR-4 autoloader, remove manual require_once calls (#27)

- Add spl_autoload_register in Plugin::register_autoloader() mapping
  WPALLSTARS\FixPluginDoesNotExistNotices\Admin\ to admin/lib/ and
  WPALLSTARS\FixPluginDoesNotExistNotices\ to includes/
- Remove manual require_once calls for Core, Admin, and Modal classes
- Rename admin/lib/admin.php -> Admin.php and modal.php -> Modal.php
  for PSR-4 filename convention (class name must match file name)
- Fix composer.json: correct namespace case (WPAllStars -> WPALLSTARS)
  and add admin/lib/ directory mapping for Admin sub-namespace
- Fix wp-fix-plugin-does-not-exist-notices.php require_once to use
  correct case (plugin.php -> Plugin.php)

Resolves #25

composer.json

@@ -15,7 +15,8 @@
     },
     "autoload": {
         "psr-4": {
-            "WPAllStars\\FixPluginDoesNotExistNotices\\": "includes/"
+            "WPALLSTARS\\FixPluginDoesNotExistNotices\\Admin\\": "admin/lib/",
+            "WPALLSTARS\\FixPluginDoesNotExistNotices\\": "includes/"
         }
     }
 }