editorconfig: Add max_line_length property

The property is supported by the most of the editors, but not all:
https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties#max_line_length

Cc: Eric Engestrom <eric@engestrom.ch>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

.editorconfig

@@ -11,6 +11,7 @@ tab_width = 8
 [*.{c,h,cpp,hpp,cc,hh}]
 indent_style = space
 indent_size = 3
+max_line_length = 78
 
 [{Makefile*,*.mk}]
 indent_style = tab