Include .editorconfig files

It looks like there are some IDE-specific configuration files here to make it easier to adopt these coding styles. However, I don't see anything for [`.editorconfig`](https://editorconfig.org/). Although it is quite simple and only able to express some of the conventions, it is very easy to use, supported by a wide range of editors, and takes care of some of the most common choices (e.g. indentation). Would you please consider adding these to your repo?

See also https://github.com/google/styleguide/issues/437

For example, a basic one (that just deals with whitespace) for C/C++ might look like something like this:

.editorconfig

```
[*.{c,cxx,cpp,h}]
indent_style = space
indent_size = 2
trim_trailing_whitespace = true
insert_final_newline = true
```