Add editorconfig

Summary:
Defaults taken from kde coding style.
This file enables a minimum style guide to be enforced across editors,
like line endings, character encoding, tabs vs spaces, indent size
and some more.
https://editorconfig.org/

Reviewers: hindenburg, tcanabrava, gszymaszek

Reviewed By: hindenburg, tcanabrava, gszymaszek

Subscribers: Omar, gszymaszek, dhaumann, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D15383