[whatwg/dom] [idea] toggleAttribute with detault value when attribute is added. (#939)

For example,

```js
// Assume we start with `el` already having the `foo` attribute:
el.toggleAttribute('foo', 'bar') // no longer has attribute `foo`
el.toggleAttribute('foo', 'bar') // now has attribute `foo` with value `bar`
el.toggleAttribute('foo', 'bar') // no longer has attribute `foo`
el.toggleAttribute('foo', 'bar') // now has attribute `foo` with value `bar`
```

- If the attribute exists, remove it. This is similar to calling `removeAttribute('foo')` while the attribute exists.
- If the attribute does not exist, add it, with the given value. This is similar to calling `setAttribute('foo', 'bar')` while the attribute does not exist.

The above code would be equivalent to this:

```js
// Assume we start with `el` already having the `foo` attribute:
el.removeAttribute('foo') // no longer has attribute `foo`
el.setAttribute('foo', 'bar') // now has attribute `foo` with value `bar`
el.removeAttribute('foo') // no longer has attribute `foo`
el.setAttribute('foo', 'bar') // now has attribute `foo` with value `bar`
```

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