🚀 Feature: Add Slack message sender with webhook - Node.js

### 🔖 Feature description

Task is to implement the Slack message sender with webhook function in Node.js. You can look at the existing [Appwrite Functions Templates](https://github.com/appwrite/templates) in another coding language.

Tasks summary:
Fork & clone Appwrite's [templates](https://github.com/appwrite/templates) repository
Create a branch feat-implement-slack-message-sender-webhook-with-nodejs
Write code for the function in folder node/slack-message-sender/.
Test the function using Appwrite, provide a video.
Write a README.md explaining what the function does, how to use it, and customize it.
Submit a pull request to our [templates](https://github.com/appwrite/templates) repository

### 🎤 Pitch

A function that would send a message to a specific channel in a specific slack organization via webhook.

Some use cases:
- Manual communication via function with the slack organization (Custom Announcements, for example)
- Sending appwrite project stats either manually or via cron task
- Sending information from 3rd party APIs

### 👀 Have you spent some time to check if this issue has been raised before?

- [X] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [X] I have read the [Code of Conduct](https://github.com/appwrite/.github/blob/main/CODE_OF_CONDUCT.md)