C#: Desugar property patterns that uses member access syntax.

[michaelnebel]

In this PR we make support for Extended property patterns.
The idea is that the new form is de-sugared into a pattern expression in the old form. The only difference will be some slightly different locations.

[hvitved]

LGTM. It would be nice to have data flow tests for this as well, e.g. in ql/test/library-tests/dataflow/fields/.

[michaelnebel]

LGTM. It would be nice to have data flow tests for this as well, e.g. in ql/test/library-tests/dataflow/fields/.

Yes, that is a good idea, but I don't think that pattern matching is really supported in the dataflow library.
As an example, the following code results in a failure of flow not being detected by dataflow library.

public struct Struct
{
    public object Field;
    public object Prop { get; init; }
    public Struct(object field, object prop) => (Field, Prop) = (field, prop);
}

private void M1()
{
    var o = Source<object>(1);
    var s = new Struct(o, o);
    if (s is Struct { Field: object p })
    {
        Sink(p); // $ hasValueFlow=1
    }
}

An issue has been created for this.