[whatwg/dom] Allow <slot> to find slottables by slottables's IDs as well (#677)

Sometimes, slottables already have IDs, so it's not necessary to require them to also define reduntant `slot` attributes. If a `slot` is able to find its slottables by slottables's IDs, we would go a step further to make the document semantically brief(_It's semantical brevity_ which is one principial cause that inspires shadow DOM, isn't it?).

I'd like suggest we add a `for` attribute to <slot> for this.

```
<slot for="id-for-the-slottable"></slot>
```

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