Add .editorconfig file

[jlumpe]

I'm trying to keep the code style consistent in my PRs but it seems to vary a lot in each file. It would be nice to have an .editorconfig file or even just a small style guide in a CONTRIBUTING file.

[ChrisRackauckas-Claude]

Triage: SciML repos now use Runic.jl for formatting. An .editorconfig is no longer the standard approach for code style. Recommend closing.