Add an .editorconfig

[Lucas2944]

Context: the repo currently has no .editorconfig. Different contributors will end up fighting over whitespace.

Goal: add a minimal .editorconfig that matches the existing style of the codebase:

root = true

[*]
indent_style = space
indent_size = 2
charset = utf-8
end_of_line = lf
trim_trailing_whitespace = true
insert_final_newline = true

[*.md]
trim_trailing_whitespace = false

Acceptance: committed at the repo root. No code changes.

Five-minute task. Good first issue for someone who's never contributed before.