[w3c/manifest] Add support for "share_target" property (#665)

Since according to @PaulKinlan's post [Chrome started supporting the Web Share Target API](https://paul.kinlan.me/web-share-target-api/) (implementing the [Draft Community Group Report 27 February 2018](https://wicg.github.io/web-share-target/)), and since no other than [Twitter added the proposed syntax in their manifest](https://mobile.twitter.com/manifest.json), it might be a great point in time to consider adding the `"share_target"` property to the Web App Manifest spec. Here is the relevant extract of Twitter's manifest, showcasing the proposed syntax with the three URL placeholders `{title}`, `{text}`, and `{url}`:

```json
"share_target": {
    "url_template": "compose/tweet?title={title}&text={text}&url={url}"
}
```

I reckon this is a bit of a 🐔 and 🥚 problem, as they both depend on each other. Happy to help with either.

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