Add _targetRef and _domRef modifiers

[MaxDesiatov]

Resolves partially #231. _targetRef is a modifier that can be used by any renderer, while _domRef is an adaptation of that for DOMRenderer. Both are underscored as they are not available in SwiftUI, and also their stability is currently not so well known to us, we may consider changing this API in the future.

Example use:

struct DOMRefDemo: View {
  @State var button: JSObjectRef?

  var body: some View {
    Button("Click me") {
      button?.innerHTML = "This text was set directly through a DOM reference"
    }._domRef($button)
  }
}

I've also fixed all known line length warnings in this PR.

[carson-katri]

Awesome 👍