[WIP] Set up Copilot instructions for repository

[Copilot]

• Analyze repository structure and technology stack
• Create .github/copilot-instructions.md with best practices
• Include project-specific context (IntelliJ Plugin, Kotlin, Gradle)
• Document build and test commands
• Document code style and conventions
• Add information about plugin architecture
• Verify the instructions file is properly formatted

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

This is an intellij ultimate plugin repository that aim to help using ngx-translate library in Angular project in the IDE.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #257

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