Support HTMLDialogElement

[halfmatthalfcat]

Support for the Dialog element is slowly gaining support among browsers and is already implemented in Chrome/Opera. Add a scalajs interface per the HTMLDialogElement spec: https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement

[armanbilge]

5 years later yet still less than 75% at caniuse :(

[Lasering]

Now is at 87.75% (with IE still being included).

For those interested in using dialog:

document.getElementById("myDialog").asInstanceOf[js.Dynamic].showModal()

[armanbilge]

Will happily take a PR for this. We changed our policy to accept any standardized APIs regardless of adoption.