add .editorconfig file for basic formatting

add .editorconfig file for basic formatting

EditorConfig is a specification to define the most basic code formatting
stuff, and it is supported by many editors and IDEs, either directly or via
plugins, including VSCode/VSCodium, Vim, emacs and more.

It allows to define formatting style related to indentation, charset, end
of lines and trailing whitespaces. It also allows to apply different
formats for different files based on wildcards, so for example it is
possible to apply different configurations to *.{c,h}, *.json or *.yaml.

In linux related projects, defining a .editorconfig might help people that
work on different projects with different indentation styles, so they
cannot define a global style. Now they will directly see the correct
indentation on every fresh clone of the project.

Add the .editorconfig file at the root of the iproute2 project with a broad
generic configuration for all file types. Then add exceptions for the file
types which follow different conventions.

See https://editorconfig.org

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: David Ahern <dsahern@kernel.org>