feat: invoke callback on onAuthStateChange (#627)

## What kind of change does this PR introduce?
* Run callback when `onAuthStateChange` is first invoked with an
`INITIAL_SESSION` event
* Fixes #313

src/GoTrueClient.ts

@@ -941,9 +941,26 @@ export default class GoTrueClient {
 
     this.stateChangeEmitters.set(id, subscription)
 
+    this.emitInitialSession(id)
+
     return { data: { subscription } }
   }
 
+  private async emitInitialSession(id: string): Promise<void> {
+    try {
+      const {
+        data: { session },
+        error,
+      } = await this.getSession()
+      if (error) throw error
+
+      this.stateChangeEmitters.get(id)?.callback('INITIAL_SESSION', session)
+    } catch (err) {
+      this.stateChangeEmitters.get(id)?.callback('INITIAL_SESSION', null)
+      console.error(err)
+    }
+  }
+
   /**
    * Sends a password reset request to an email address.
    * @param email The email address of the user.

src/lib/types.ts

@@ -23,6 +23,7 @@ export type Provider =
 export type AuthChangeEventMFA = 'MFA_CHALLENGE_VERIFIED'
 
 export type AuthChangeEvent =
+  | 'INITIAL_SESSION'
   | 'PASSWORD_RECOVERY'
   | 'SIGNED_IN'
   | 'SIGNED_OUT'