Allow to generate unions instead of TS enums in Typescript

[akozhemiakin]

Resolves #35

Now for unit enums, you can choose to generate a TypeScript union type instead
of a TypeScript enum.

#[typeshare(ts_union)]
pub enum UnitEnum {
    VariantA,
    VariantB,
    VariantC,
}

This would generate the following TypeScript code:

export type UnitEnum = "VariantA" | "VariantB" | "VariantC";

[Tehnix]

Any chance of reviving this? If there's interest in getting this merged I wouldn't mind taking a look at it