Add libeditorconfig

EditorConfig is a tiny (less than 30 KiB) optional dependency for KTextEditor, which allows apps like Kate or KDevelop to automatically use project-specific configuration (like indent width or character encoding). As far as I know it cannot be easily included by individual apps, since that would require each of them to rebuild KTextEditor as well.

I tested this change on the 5.15-22.08 runtime and confirmed to be working with KWrite and KDevelop, not sure if I should open this merge request against that branch, or if it's ok to include it in the new 5.15-23.08 version.