Add method to indicate an event handler should enforce value consistency

[SteveSandersonMS]

In other words, a way to tell Blazor that a given event handler is part of a two-way binding even if you're not using @bind.

It would probably look like this:

<input value="@MyValue" @onchange="MyHandler" @onchange:enforceConsistency />

... or maybe @onchange:sync or @onchange:twoWay. The implementation is simply to emit a builder.SetUpdatesAttributeName call for whatever is the "value" attribute for this element type (e.g., on <input type=checkbox> it would call builder.SetUpdatesAttributeName("checked");).

For more details about the scenario and justification for this design, see #17099 (comment)

[JvanderStad]

The @onchange event handler receives a Microsoft.AspNetCore.Components.ChangeEventArgs instance.

namespace Microsoft.AspNetCore.Components
{
    //
    // Summary:
    //     Supplies information about an change event that is being raised.
    public class ChangeEventArgs : EventArgs
    {
        public ChangeEventArgs();

        //
        // Summary:
        //     Gets or sets the new value.
        public object Value { get; set; }
    }
}

Can't we use the Value-setter to reflect changes?

public void ChangeEvent(ChangeEventArgs args)
{
	args.Value = "new value";
}