📝 Document .editorconfig as oxfmt fallback

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.

.vscode/extensions.json

@@ -3,6 +3,7 @@
         "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
         "typescriptteam.native-preview",
-        "sandy081.todotasks"
+        "sandy081.todotasks",
+        "EditorConfig.EditorConfig"
     ]
 }