[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 EnzDev/idea-ngx-translate-autocomplete#257

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