Event binding guidance improvements

[guardrex]

Fixes #35976

@danroth27 @javiercn ... Holding until we figure out the right guidance to provide.

I probably need a code example of the "mapping" part of the suggestion. The direction that I thought you were pitching is (and I'll use onkeyup as my example because onkeydown is probably best served by just oninput) ...

using Microsoft.AspNetCore.Components;

namespace {APP NAMESPACE};

[EventHandler("onkeyup", typeof(ChangeEventArgs),
    enableStopPropagation: true, enablePreventDefault: true)]
public static class EventHandlers
{
}

... but that with ...

<input @bind="boundValue" @bind:event="onkeyup" />

... throws an unhandled exception. Perhaps, it's because onkeyup is already mapped as a keyboard event argument.

One simple approach for this scenario is to use @on delegate event handler syntax ...

<input @onkeyup="@((KeyboardEventArgs args) => boundValue = args.Key)" />

... but that's not the ask on the issue, and it's clunky compared to @bind:event="onkeyup" syntax (if that can be made to work).

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