[enh] add .editorconfig

EditorConfig [1] helps maintain consistent coding styles for multiple developers
working on the same project across various editors and IDEs.

The EditorConfig specification [2] support is pre installed in common IDEs [3]
and plugins for many others are available [4].

[1] https://editorconfig.org
[2] https://editorconfig-specification.readthedocs.io/
[3] https://editorconfig.org/#pre-installed
[4] https://editorconfig.org/#download



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>