Support trailing comma in various comma-separated lists.

Today it is not legal to do the following:

```vb
Public Sub Foo(
Arg1 As Integer,
Arg2 As String, '<--- compiler complains here.
)

Dim MyArray = {
"one",
"two", '<--- compiler complains here.
}

Dim person = New CPerson() With {
.Name = "foo",
.Age = 100, '<--- compiler complains here.
}
End Sub
```

The comma-seperated list of arguments, array elements and field initialisers all have a trailing comma in the above code and the compiler reports an error saying an identifier is required.

Supporting this appears to be a small syntax change which would help dramatically when editing code using commands such as `DuplicateLine` and `MoveLineUp` and `MoveLineDown` which always results in a syntax error when touching the last item in comma-seperated list. For example, starting with this valid code:

```vb
Dim MyArray = {
"one",
"two"
}
```

If you execute the `MoveLineUp` editing command on the line containing string `"two"`, you end up with the following which is a syntax error today:

```vb
Dim MyArray = {
"two" '<-- comma needed here.
"one", '<-- comma not allowed here.
}
```

I've justed tried this in C#, it works for the array and object initialiser but not on method declarations:

```c#
public class Class1
{
public string name;
public int age;

private string[] myarray = {
"one",
"two", //<--- this is OK.
};

private Class1 foo = new Class1() {
name = "foo",
age = 12, //<--- this is OK.
};

private void Foo(
int Arg1,
int Arg2, //<--- this breaks.
) {

}
}
```

CC: @AnthonyDGreen