Add comprehensive GitHub Copilot instructions for firebase-kotlin-sdk

[Reedyuk]

This pull request adds a comprehensive set of GitHub Copilot instructions for the firebase-kotlin-sdk project. The new documentation outlines the project's architecture, design principles, serialization guidelines, API compatibility goals, platform-specific notes, documentation and code style requirements, binary API validation process, testing instructions, and explicit guidance on practices to avoid.

This will help maintain consistency and quality across contributions, especially when using GitHub Copilot.

Key additions and themes:

Project Structure and Design Principles

• Provides an overview of the SDK's multiplatform architecture, module structure, and Kotlin-first design principles, including the use of suspend functions, Flow, default arguments, infix notation, and operator overloading.

Serialization and API Compatibility

• Details serialization requirements using kotlinx.serialization and outlines strategies for maintaining near-binary compatibility with the Firebase Android SDK, including naming conventions and API design.

Platform-Specific Guidance

• Explains platform-specific considerations for Android, iOS, JVM/Desktop, and how to access underlying native SDKs via extension properties in the appropriate source sets.

Documentation, Code Style, and Testing

• Establishes requirements for KDoc on all public APIs, adherence to IntelliJ Kotlin code style, use of formatting/linting tools, binary API validation, and instructions for running tests across platforms. (.github/copilot-instructions.mdR1-R214

[Copilot]

Pull request overview

Adds repository-specific GitHub Copilot guidance for the firebase-kotlin-sdk to steer generated changes toward the project’s Kotlin-first, multiplatform architecture and API-compatibility goals.

Changes:

• Introduces a new .github/copilot-instructions.md with architecture, API, serialization, style, and testing guidance for contributors/Copilot.
• Documents Kotlin-first design principles (suspending APIs, Flow usage, default args over builders) and binary API validation expectations.

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