C# 14: Support the field keyword.

[michaelnebel]

In this PR we make support for the C# 14 field keyword in properties. Using the keyword field in property accessors implicitly make a synthetic backing field.
That is, you can declare

public class C
{
    public object Prop1
    {
        get { return field; }
        set { field = value; }
    }
}

Some notes on the implementation.

• An important implementation detail is that PropertyField objects are hashed on the (field,property) pair and not only on the field itself. The symbol visitor is being applied to all backing fields, which means that Field objects are created for backing fields as well - however, the DB is not populated based on these as they are implicitly declared.
• An auto-property has a similar backing field, which we are not extracting.
• We only extract the property backing field, if it is referenced by the use of the field keyword. That is, we only extract the backing field, if we are encountering an expression with FieldExpression kind.