Interop Gen: Support Literal Types in TS (i.e `null`, `true`, `false`)

[nikeokoronkwo]

This issue is to add support for generating Dart JS Interop Code for types that are a combination of literal types:

export const alwaysTrue: true;
export const alwaysFalse: false;
export const nullable: string | null;

export interface Literals {
  trueBoolean: true;
  stringOrfalseBoolean: string | false;
}

This would most likely mean matching with the literal if possible and using its base type (this means treating true and false as boolean if in a union, or if as a single type, maybe give the value as is without making an external call)

[nikeokoronkwo]

Completed by #404