Add .editorconfig to share code-style settings among IDEs

Problem

Code editor default settings sometimes deal with scapes and newlines in different formats (e.g. whitespaces are saved as spaces or whitespaces) and new lines are saved as LF or CR LF. MRs sometimes gets more difficult to read due to these unwilling whitespace change.

An initial attempt was in this https://gitlab.com/gitlab-org/security-products/analyzers/api-fuzzing-src/-/merge_requests/637

Proposal

• Set minimal .editoconfig
  • Set policy for whitespaces
• Try use dotnet format command (make suer it only applies whitespace changes)
• Add automation (lint) en sure .editoconfig format correct.