feat(medusa-plugin-mailchimp): Migrate to TS + add update endpoint (#4040)

* - Refactored the .js files to .ts files
- Added typing
- Used 'WrapHandler' util instead of specific middleware in plugin
- Added additional PUT method to the /mailchimp/subscribe endpoint.
- Updated documentation where relevant.

* Added changeset

* Updated yarn.lock

* Used camelcase for method args in services/mailchimp.ts

---------

Co-authored-by: Tom Rupke <tom@adapptive.nl>
Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>

packages/medusa-plugin-mailchimp/README.md

@@ -22,32 +22,31 @@ Manage newsletter subscriptions in your commerce application with Mailchimp.
 
 1\. Run the following command in the directory of the Medusa backend:
 
-  ```bash
-  npm install medusa-plugin-mailchimp
-  ```
+```bash
+npm install medusa-plugin-mailchimp
+```
 
 2\. Set the following environment variables in `.env`:
 
-  ```bash
-  MAILCHIMP_API_KEY=<YOUR_API_KEY>
-  MAILCHIMP_NEWSLETTER_LIST_ID=<YOUR_NEWSLETTER_LIST_ID>
-  ```
+```bash
+MAILCHIMP_API_KEY=<YOUR_API_KEY>
+MAILCHIMP_NEWSLETTER_LIST_ID=<YOUR_NEWSLETTER_LIST_ID>
+```
 
 3\. In `medusa-config.js` add the following at the end of the `plugins` array:
 
-  ```js
-  const plugins = [
-    // ...,
-    {
-      resolve: `medusa-plugin-mailchimp`,
-      options: {
-        api_key: process.env.MAILCHIMP_API_KEY,
-        newsletter_list_id: 
-          process.env.MAILCHIMP_NEWSLETTER_LIST_ID,
-      },
+```js
+const plugins = [
+  // ...,
+  {
+    resolve: `medusa-plugin-mailchimp`,
+    options: {
+      api_key: process.env.MAILCHIMP_API_KEY,
+      newsletter_list_id: process.env.MAILCHIMP_NEWSLETTER_LIST_ID,
     },
-  ]
-  ```
+  },
+]
+```
 
 ---
 
@@ -55,11 +54,11 @@ Manage newsletter subscriptions in your commerce application with Mailchimp.
 
 1\. Run the following command in the directory of the Medusa backend to run the backend:
 
-  ```bash
-  npm run start
-  ```
+```bash
+npm run start
+```
 
-2\. Use the `/mailchimp/subscribe` endpoint or the `MailchimpService` to subscribe to the newsletter.
+2\. Use the POST or PUT `/mailchimp/subscribe` endpoint or the `MailchimpService` to subscribe to the newsletter.
 
 ---