Goal

Allow the use of #[ts(flatten)] with unit structs struct Foo; and empty structs struct Foo {} but not empty tuple structs struct Foo(). This matches the behavior of #[serde(flatten)]

The use of the type { } when flattening these structs does not cause the {} type to appear in typescript because of our .replace(" } & { ") which we added back when we first changed the implementation of flatten to use type intersections, which means these structs simply disappear from the ts type as though #[ts(skip)] was used instead, which also matches serde behavior.

Allowing flatten on struct Foo; (no curly braces) broke the generation of internally tagged enums because it caused an interaction with the struct version of #[ts(tag = "...")], but this ended up making the enum generation code simpler as it removed an unnecessary match expression and the need to manually add the internal tag, as the struct version of this attribute does it instead

Also added a validation rule to prevent the use of internal tagging (#[ts(tag = "...")] without #[ts(content = "...")]) when the enum contains tuple variants,
Closes #473

Changes

How did you go about solving the problem?

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.