Add [AlsoNotifyCanExecuteFor] attribute

[Sergio0694]

Contributes to #8

This PR adds the [AlsoNotifyCanExecuteFor] attribute.

API breakdown:

namespace CommunityToolkit.Mvvm.ComponentModel
{
    [AttributeUsage(AttributeTargets.Field, AllowMultiple = true, Inherited = false)]
    public sealed class AlsoNotifyCanExecuteForAttribute : Attribute
    {
        public AlsoNotifyCanExecuteForAttribute(string commandName);
        public AlsoNotifyCanExecuteForAttribute(string commandName, params string[] otherCommandNames);
        public string[] CommandNames { get; }
    }
}