Add an analyzer for [field: ObservableProperty] uses #734

The [ObservableProperty] attribute is only valid on field declarations. Trying to use it on a backing field for a property will not cause errors, as the source generator will simply skip that syntax node, but it will produce no diagnostics to tell the users what's wrong. There should be a new diagnostic analyzer to produce the appropriate diagnostic in these cases.

Example

Consider this snippet:

public partial class MyViewModel : ObservableObject
{
    [field: ObservableProperty]
    public string? Name { get; private set; }
}

This should emit a diagnostic on that attribute use.