feat: replace Client.authFromCode() with Client.fromAuthCode()

`authFromCode()` was a mutating method, which goes against the API
design vision of snoots. It has been replaced with `fromAuthCode()`,
which is a static factory function. This is an important step towards
ensuring the Client object is internally immutable.

BREAKING CHANGE: `Client.authFromCode()` has been replaced with
`Client.fromAuthCode()`.

examples/oauth/index.ts

@@ -50,13 +50,14 @@ app.get("/auth", async (req, res) => {
     return;
   }
 
-  // Create a new Reddit client.
-  const client = new Client({ userAgent, creds });
-
   try {
-    // Authenticate with the code we were given. Note that the redirectUri here
-    // MUST match the uri given to `getAuthUrl`, or else this will fail.
-    await client.authFromCode(code, redirectUri);
+    // Create a new Client from the given auth code. Note that the redirectUri
+    // here MUST match the uri given to `getAuthUrl`, or else this will fail.
+    const client = await Client.fromAuthCode(
+      { userAgent, creds },
+      code,
+      redirectUri
+    );
 
     // Get and display the user's information so we know it worked!
     const user = await client.users.fetchMe();

src/client.ts

@@ -209,20 +209,34 @@ export default class Client {
   }
 
   /**
-   * Authorize this client from an OAuth code.
+   * Create a client from an OAuth code.
    *
+   * @param opts The Client options.
    * @param code The OAuth code.
    * @param redirectUri The redirect URI. This ***must*** be the same as the uri
-   * given to {@link OAuth.getAuthUrl}.
+   * given to {@link makeAuthUrl}.
    *
    * @returns A promise that resolves when the authorization is complete.
    */
-  async authFromCode(code: string, redirectUri: string): Promise<void> {
-    const creds = this.creds;
+  static async fromAuthCode<Self extends typeof Client>(
+    this: Self,
+    opts: Required<Omit<ClientOptions, "auth">>,
+    code: string,
+    redirectUri: string
+  ): Promise<InstanceType<Self>> {
+    const creds = opts.creds;
     if (!creds) throw "No creds";
 
-    this.token = await tokenFromCode(code, creds, this.userAgent, redirectUri);
-    this.auth = { refreshToken: this.token.refresh! };
+    const client = new this(opts);
+    client.token = await tokenFromCode(
+      code,
+      creds,
+      client.userAgent,
+      redirectUri
+    );
+    client.auth = { refreshToken: client.token.refresh! };
+
+    return client as InstanceType<Self>;
   }
 
   /**