Recommend `EditorConfig.EditorConfig` in .vscode/extensions.json so editors
without native `.editorconfig` support pick it up, and add an "Editor
configuration" section to CONTRIBUTING.md: `.editorconfig` is an
editor-compatibility fallback for the file types oxfmt does not format
(shell scripts, dotfiles, the commit-message template, git's
`COMMIT_EDITMSG` buffer), with `.oxfmtrc.json` authoritative where both
apply. `.editorconfig` itself is unchanged.