use props when passing data to custom elements (#875)

[Rich-Harris]

This changes the behaviour of attributes on custom elements — previously, data would always be passed to custom elements using setAttribute, which only works for strings.

Now, it uses a simple heuristic: if the prop exists on the custom element, it is set as a prop, otherwise it's an attribute. (This is similar to what Preact does.)

So in this case...

<fancy-list items={things}/>

...as long as items is the name of a property on a FancyList instance (whether or not it had previously been set), the value of items inside the component will be an array rather than the dreaded "[object Object]".

Typically, custom elements use accessors for properties that their consumers should be able to read and write...

class FancyList extends HTMLElement {
  ...
  get items() {
    // ...
  }

  set items(items) {
    // ...
  }
}

...and that's similar to the code Svelte generates for custom elements, so this ought to be a reliable heuristic. Meanwhile, unexpected props still become attributes, allowing this sort of thing:

<fancy-list style="font-family: 'Comic Sans MS'" items={things}/>

With this PR, we pass all the tests on https://custom-elements-everywhere.com —

— which is better than React, Preact, and, err... Polymer, the custom element framework 😳