fix: use package type discoverable by phpstan

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>

composer.json

 {
 	"name": "wpdesk/phpstan-rules",
 	"description": "Meta package with PHPStan configuration for WP Desk projects",
-	"type": "metapackage",
+	"type": "phpstan-extension",
 	"require": {
 		"phpstan/phpstan": "^1.10",
 		"phpstan/extension-installer": "^1.3",
@@ -18,7 +18,7 @@
 	],
 	"config": {
 		"allow-plugins": {
-			"phpstan/extension-installer": true
+			"phpstan/extension-installer": false
 		}
 	},
 	"extra": {