Add .editorconfig to properly detect indentation in editors

Editorconfig is a standard of defining indentation style and other
settings such as newline types for different file types in a project.

See https://editorconfig.org/

It is achieved by introducing a simple .editorconfig dotfile with
glob expressions and settings per fiile type. Editors then can read
this file and decide which indentation style they should use for a
given project file.