[Proposal]: struct members assign operator `<=`

I suggest to solve [this issue]( https://github.com/dotnet/vblang/issues/511) and [this](https://github.com/dotnet/vblang/issues/487) by generally solving the whole issue for structures, by adding the `<=` operator:
```VB.NET
Dim l = new List(Of (X As Integer, Y As Integer)
l.Add((1,1))
l(0).X <= 2
```

Which is lowered to:
```VB.NET
var temp = l(0)
temp.x = 2
l(0) = temp
```