Cleanup public API

[EamonNerbonne]

• Avoid need for IObjectStringifier when it's mostly equivalent to a few bools in the config.
• Make a type that's nice for using static usage, particularly in a global imports section
• Make config types/api more discoverable to make special case usage easier.
• use modern C# features where applicable (i.e. nullability/records).
• Try and use obsoletion indicators to make it easy to migrate from the old api to the new one, ideally with few or no breaking changes.
• Update readme with examples of this new API, including features like @girlpunk's Option disable literal strings #245
• Push to nuget as 4.0.0