Add .editorconfig shared editor configuration

EditorConfig is a format for specifying configuration information, such as indentation, supported by many common editors and IDEs. A benefit is encouraging consistency across editors from a single shared configuration file, rather than editor-specific modelines in each file or requiring each developer to configure their own project-specific settings.

This commit adds a .editorconfig file with settings that match the most common current indent styles. It is not intended to proscribe any changes to current style, but to encourage consistency going forward.

Thanks for considering,
Kevin