Templating: enable making Var holes without a corresponding input element

[Tarmil]

This would be particularly useful in a server-side scenario where you just want a small client function to toggle a value, for example:

<button ws-onclick="ShowPanel">Show</button>
<button ws-onclick="HidePanel">Hide</button>
<div id="panel" class="${PanelClass}"></div>

MyTemplate()
    .HidePanel(fun e -> e.Vars.PanelClass := "hidden")
    .ShowPanel(fun e -> e.Vars.PanelClass := "")
    .Doc()

Right now the above doesn't work, because PanelClass can only be bound as a string or View<string>, not Var<string>. It is only doable by adding something like this in the HTML, but it's quite hackish:

<input type="hidden" ws-var="PanelClass">

I'm not sure what form this would take, but it needs to be known statically. Ideas:

• Use a special syntax when declaring the hole itself, such as ${!Name}.

• Pass a list of vars when declaring the provider:

  type MyTemplate = Template<"index.html", Vars = "PanelClass,SomeOtherVar">