Closes #207.

This PR introduces Mod[IO, Elem] which reifies modifier, as well as a .toMod syntax.

This would make it possible to write:

def MyComponent(mod: Mod[IO, HtmlElement[IO]]) = 
  div(
    cls := "decoration",
    mod
  )

instead of

def MyComponent[M](mod: M)(using Modifier[IO, HtmlElement[IO], M]) = 
  div(
    cls := "decoration",
    mod
  )

and generally make it easier to pass modifiers around.

I'm not entirely sold on this yet, since it moves in the direction that Modifier should not be a typeclass, which in turn was motivated so that a .toMod extension wouldn't be needed for stuff like String or HtmlElement. But if we're going to do that anyway, then maybe we should change the encoding.

h/t @kubukoz, cc @yurique