editorconfig: add simple top-level file

editorconfig: add simple top-level file

Add a simple top-level .editorconfig file to manage common attributes
such as indentation style, trailing whitespace and newline at end of
file. The format is wide spread and has support for nearly every editor
out there - see https://editorconfig.org/ for more.

Majority of the project is C - which uses tabs, although there are some
CMake files using 2 space indent and shell scripts - predominantly using
4 space indent.

This makes it harder for casual contributors to butcher things :-)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>