[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.

未指定檔案或測試。首先定位 issue 中引用的現有 rawvalue 支援，並檢查所示 TypeScript 聯合型別如何對應至 Swift；釐清應如何表示變體之間的執行階段變更。完成的標準是 Example 聯合型別和 example() 簽章能夠正確互通，包括 Foo 和 Bar 之間的動態變更。