[BridgeJS] Support union types

Union types (other than rawvalue which are already done) should be supported. The best way to do this is unclear to me so this issue functions as a discussion point.

Example:

export type Foo = { foo: string } 
export type Bar = { bar: string }
export type Example = Foo | Bar
export function example(): Example

Could maybe generate:

@JSClass struct Foo {
    @JSGetter var foo: String
    @JSSetter func setFoo(_ value: String) throws(JSException)
}
@JSClass struct Bar {
    @JSGetter var bar: String
    @JSSetter func setBar(_ value: String) throws(JSException)
}
@JSClass enum Example {
    case foo(Foo)
    case bar(Bar)
}
@JSFunction func example() throws(JSException) -> Example

Note that ts unions can change dynamically eg if I set foo to null and bar to a string, the type changes from Foo to Bar, so idk if that works with an enum like this in Swift.

未指定文件或测试。首先定位 issue 中引用的现有 rawvalue 支持，并检查所示 TypeScript 联合类型如何映射到 Swift；明确应如何表示变体之间的运行时变化。完成的标准是 Example 联合类型和 example() 签名能够正确互操作，包括 Foo 和 Bar 之间的动态变化。