chore: add editorconfig

[michalmaj]

Summary

Adds an EditorConfig file for basic editor-level consistency.

Why

The repository should keep simple formatting conventions consistent across editors without adding a new tool or workflow step.

Changes

• Adds .editorconfig
• Sets UTF-8 encoding
• Uses LF line endings
• Ensures final newlines
• Trims trailing whitespace by default
• Preserves trailing whitespace in Markdown
• Defines indentation for Python, TOML, YAML, and JSON files

Type of change

• Documentation
• Project configuration
• New feature
• Tests
• CI/CD
• Refactoring
• Bug fix

Checklist

• The change is small and focused
• The pull request title is clear
• I updated documentation if needed
• I added or updated tests if needed
• I ran local checks if applicable

Notes

Local checks:

uv run ruff check .
uv run ruff format --check .
uv run pytest