Add an .editorconfig file

Add an .editorconfig file

It's currently not possible to configure our indentation scheme for
continuation lines (i.e. use 1-3 spaces to align with the upper line).
There is an issue open regarding this, see [1].  So we can't run e.g.
eclint over our codebase to detect issues without getting a lot of
false positives.

The main trigger was that this sets the preferred tab width in GitHub's
code browser.

[1] https://github.com/editorconfig/editorconfig/issues/323

References #3111.