Indexer setters don't work with optional parameters

F#'s Indexer setters don't work with optional parameters, but C#'s do.

Related #9369

**Repro steps**

```fsharp
open System.Runtime.CompilerServices

type A() =
member this.Item
with get (key: string, []?path: string, []?line: int) =
printfn "%A" (key, path, line)
"_"

// error FS1212: Optional arguments must come at the end of the argument list, after any non-optional arguments
and set (key: string, []?path: string, []?line: int) (value: string) =
printfn "%A" (key, path, line)

let t = A()
t["a"]
```

**Related information**

Provide any related information (optional):

* .NET Runtime kind: .NET 11
* Editing Tools: dotnet fsi

Start by reproducing the supplied indexer-setter example in dotnet fsi and compare its behavior with the getter and the stated C# behavior. Trace the compiler handling for optional parameters in indexer setters; done means the example compiles and the setter accepts optional parameters without FS1212.

Written by the indexing model from the issue text.