Dpg - Generate anonymous models inline

[joheredi]

TypeSpec allows defining anonymous models:

model Foo {
   bar: {
       baz: string;
    }
}

TypeScript allows generating this so we should generate similar to the TypeSpec definition

interface Foo {
   bar: {
       baz: string;
    }
}

This will solve some issues identified:

• Problem with TypeScript Emitter not generating anonymous or inline objects
• Issue with inline models not serializing. Workaround: Handwrite model.
• output model for Error (=> ErrorModel) is not being generated. Workaround: Handwrite model which also solves the isUnexpected issue.
• Issue with isUnexpected not narrowing down types in functions like _createFineTuningJobDeserialize.