devtools: add EditorConfig file

devtools: add EditorConfig file

EditorConfig is a file format and collection of text editor plugins for
maintaining consistent coding styles between different editors and IDEs.

Initialize the file following the coding rules in
doc/guides/contributing/coding_style.rst,
doc/guides/contributing/documentation.rst and
doc/guides/contributing/patches.rst.

In order for this file to be taken into account (unless they use an
editor with built-in EditorConfig support), developers will have to
install a plugin.

Note: The max_line_length property is only supported by a limited number
of EditorConfig plugins. It will be ignored if unsupported.

Add this new file in MAINTAINERS in the "Developers and Maintainers
Tools" section.

Link: https://editorconfig.org/
Link: https://github.com/editorconfig/editorconfig-emacs
Link: https://github.com/editorconfig/editorconfig-vim
Link: https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties#max_line_length
Signed-off-by: Robin Jarry <robin.jarry@6wind.com>