Formalized coding standard.

[webbju]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

• Initial .editorconfig to inform IDEs with coding standard.
  • This is largely cherry-picked from Add a way to lint and generate #ifdefs around UEventAPI calls. #15.
• Fix import order consistency (third-party imports be sorted bottom).
• Fix sources using dotnet format, which applies the requirements of .editorconfig.

[arturjpv]

Ok!