chore: add root .editorconfig aligned with create-python-app

[ulises-jeremias]

Context

create-python-app and website ship a root .editorconfig. The cpa-templates bank repo does not, so editor defaults can drift across contributions.

Steps

1. Copy baseline rules from Create-Python-App/create-python-app/.editorconfig.
2. Adapt for file types used in this bank (Python, JSON, YAML, Markdown, Makefile).
3. Add the file at the repo root.
4. Open a PR with a chore: commit.

Acceptance criteria

• Root .editorconfig exists
• Python uses spaces, indent 4; JSON/YAML indent 2
• insert_final_newline = true and trim_trailing_whitespace = true
• No CI/script changes required

Files

• .editorconfig (create)
• Reference: create-python-app/.editorconfig

How to verify

Open a Python and a JSON file in an editor that respects EditorConfig and confirm indentation hints match.