mutable variables become immutable

[Booksbaum]

export declare module M {
    var v2: number;
    let l1: number;
    const c1: number;
}

==>

module M =
    type [<AllowNullLiteral>] IExports =
        abstract v1: float
        abstract l1: float
        abstract c1: float

v1 and l1 should be mutable:

module M =
    type [<AllowNullLiteral>] IExports =
        abstract v1: float with get,set
        abstract l1: float with get,set
        abstract c1: float