Feature request: Support optional flattened enums

[4dawit]

Is your feature request related to a problem? Please describe.
Currently, inline_flattened() under impl<T: TS> TS for Option<T> panics and says this can't be flattened

ts-rs/ts-rs/src/lib.rs

Line 822 in 858f413

panic!("{} cannot be flattened", <Self as crate::TS>::name())

Describe the solution you'd like
From this example:

#[derive(TS, Serialize)]
#[serde(rename_all = "camelCase")]
enum Enum {
    FirstOption(String),
    SecondOption(bool)
}

#[derive(TS, Serialize)]
struct T {
    a: String,
    #[serde(flatten)]
    flattened_field: Enum // I want this to be Option<Enum>
}

Non optional flattened enums get parsed to something like:

{ a: string } & ({ "firstOption": string } | { "secondOption": bool })

My guess is that adding ? to the enum keys creates intended behavior (either accepts one option because of | or allows undefined if none are given):

{ a: string } & ({ "firstOption"?: string } | { "secondOption"?: boolean })