Add .editorconfig file
Many project in the freedesktop graphics space use editorconfig files as a way to standardize style in an editor agnostic way. This one configures all of the files current in libglvnd (I looked at the .c and .h files and they seem to all use 4 spaces for indent), and meson. Because it seemed everything used 4 spaces I just set that to the default and overrode as necessary.