Add root-level EditorConfig for contributor consistency

### Describe the solution you'd like

The repository currently relies on formatter tooling, but does not expose a root-level `.editorconfig` file for consistent editor behavior across contributors.

Adding an `.editorconfig` would help standardize:
- indentation
- line endings
- trailing whitespace handling
- newline behavior

This would improve contributor consistency across different editors and IDE environments.

### Describe alternatives you've considered.

Contributors can currently rely on formatter tooling manually, but a repository-level EditorConfig would provide editor-agnostic consistency.