Add Razor compiler support for the @formname attribute

[MackinnonBuck]

Blazor will support a new @formname attribute for use on <form> elements. For example:

<form method="post" @onsubmit="() => _submitted = true" @formname="named-form-handler">
    <AntiforgeryToken />
    <InputText @bind-Value="Parameter" />
    <input id="send" type="submit" value="Send" />
</form>

@if (_submitted)
{
    <p id="pass">Hello @Parameter!</p>
}

@code{
    bool _submitted = false;

    [SupplyParameterFromForm(Handler = "named-form-handler")] public string Parameter { get; set; } = "";
}

This is currently handled in Blazor by doing a special check for the @formname attribute name in RenderTreeBuilder.AddAttribute, but we want this to be handled by the Razor compiler instead.

When encountering @formname="named-form-handler", the Razor compiler should output:

__builder.AddNamedEvent("onsubmit", "named-form-handler");

[chsienki]

@jjonescz Feel free to take this one, we might need to do a bit of Language Design but I think most of it is ready to go.

[SteveSandersonMS]

Note that as per https://github.com/dotnet/Razor-Language-Design/discussions/3#discussioncomment-6719692, we removed the sequence parameter from AddNamedEvent. I've also updated the spec above to match.