Also added a rule to silence StyleCop complaining about StaticReadonlyFieldsMustBeginWithUpperCaseLetter to match what we already have configured for the IDE.
Having this set to "none" disabled the IDE's option to add braces, whereas "silent" lets it do it on the user's request while still not suggesting it on its own.
- The DEFAULT_PRIVATE_MODIFIER behaviour is now handled by the .editorconfig file via `dotnet_style_require_accessibility_modifiers = omit_if_default:warning`.
Also added `dotnet_diagnostic.IDE0040.severity = warning` there to raise compile-time errors in the CI.
- The field naming conventions seem to already be covered by (some) analyzer rules (checked in both VS and VSCode) - IDE1006/SA1306 and SA1307.
