Add Ballerina Slack Library

[RDPerera]

Introduction to Ballerina:
Ballerina is an open-source, cloud-native programming language specifically designed for integration and service-oriented architecture. Built to simplify the challenges of connecting diverse APIs, systems, and services, Ballerina combines both the ease of use of modern programming languages and the robustness required for seamless integrations. Its unique features include a syntax designed around API-first principles, built-in data manipulation, and distributed system capabilities. Learn more at ballerina.io.

About the Ballerina Slack Library:
The Ballerina Slack Library is a dedicated integration library for the Slack API. It provides developers with streamlined access to Slack's extensive API capabilities directly within the Ballerina programming environment. Key features include:

• Simplified access to Slack's messaging, workspace, and file-sharing features.
• Support for Slack's OAuth2 authentication flow.
• High-level abstractions for interacting with Slack's real-time messaging and web API.
• The library is actively maintained and available at Ballerina Central.

Why Include Ballerina in Awesome Slack?

Adding the Ballerina Slack Library to the list will:

• Expand the ecosystem of tools available to Slack developers.
• Enable developers familiar with Ballerina to build Slack integrations quickly and efficiently.
• Strengthen collaboration between Slack and the Ballerina open-source community.

[RDPerera]

@raycharius Can you please review this ?