Start subscriptions with a free trial

trial_days, trial_ends_at and trial_once when creating subscriptions (app and
user token) and per subscription of checkout sessions. isTrial() on
subscriptions, the subscription.trial_will_end event type, and the
trial_already_used error code in the docs.

src/Id/Concerns/HasSubscriptionStatus.php

@@ -48,6 +48,14 @@ trait HasSubscriptionStatus
         return in_array($this->status(), [SubscriptionStatus::TRIAL, SubscriptionStatus::ACTIVE, SubscriptionStatus::PAUSED, SubscriptionStatus::CANCELLING], true);
     }
 
+    /**
+     * In its free trial: running, nothing billed until trial_ends_at.
+     */
+    public function isTrial(): bool
+    {
+        return $this->status() === SubscriptionStatus::TRIAL;
+    }
+
     /**
      * Paused: runs until the paid period ends, then expires (resume() to continue).
      */