Add .editorconfig to SBRP

[ellahathaway]

It would be helpful to add an .editorconfig to source-build-reference-packages repo.

.editorconfigs: https://github.com/dotnet/maintenance-packages/blob/5fa252991a931aded571a63b12159de2040f6bc9/.editorconfig, https://github.com/dotnet/installer/blob/main/.editorconfig

Related MSBuild property: https://github.com/dotnet/maintenance-packages/blob/5fa252991a931aded571a63b12159de2040f6bc9/Directory.Build.props#L15