[TS] invalid TS emited for mutable structs

[goswinr]

When targeting Typescript mutable fields on structs cause the struct to be wraped in FSharpRef<_> , but then the access to the field fails to compile
This happens only when there is mutable after val

[<Struct>]
type R =

    val mutable X: float

    new(x: float) = { X = x }

    member this.IsEmpty() =
        // emits invalid TS
        // Property 'X' does not exist on type 'FSharpRef<Rt>'.
        this.X < 0.0

compiles to

export class R extends Record implements IEquatable<R>, IComparable<R> {
    X: float64;
    constructor(X: float64) {
        super();
        this.X = X;
    }
}

export function R__IsEmpty(this$: FSharpRef<R>): boolean {
    return this$.X < 0; // Property 'X' does not exist on type 'FSharpRef<Rt>'.
}

why is FSharpRef<_> needed here?

Tested on 5.rc5 and in REPL

[goswinr]

thanks @ncave for the quick fix,
I tested 142afe4 .
this works for members, but still errors on override like this.ToString()

[<Struct>]
type R =

    val mutable X: float

    new(x: float) = { X = x }

    override this.ToString() = $"{this.X}" 

still emits FSharpRef<_>

    toString(): string {
        const this$: FSharpRef<R> = this;
        return `${this$.X}`;
    }

[ncave]

@goswinr Thanks for contributing the issue. Fixed in #4466.