Support bind:value for selectmenu

[tbdrz]

Describe the problem

When trying to use bind:value for the HTMLElement selectmenu (link), i get an invalid-binding error message: 'value' is not a valid binding on elements.

Describe the proposed solution

Add support for the selectmenu element

Alternatives considered

<script>
  let selected;
</script>

<selectmenu on:change={(e) => (selected = e.target.value)}>
  <option>Option 1</option>
  <option>Option 2</option>
  <option>Option 3</option>
</selectmenu>

Importance

nice to have

[jrmoynihan]

Might be a while for this one. I suspect the maintainers will wait until it lands in the HTML spec ("use the platform" is the mantra around Svelte).

So far, not much movement on implementation outside of the experimental flag in Chromium:
https://chromestatus.com/feature/5737365999976448