Map WebKit positions to web-features IDs

The [web-features repository](https://github.com/web-platform-dx/web-features) maintains a growing list of web platform features. This list acts as a hub for various web data such as browser-compat-data, Can I Use, specs, WPT, Chrome's usage counters, etc. The most useful thing the web-features repo offers right now is a summary of browser support and a Baseline status per feature. The Baseline status, in particular, is used on the MDN Web Docs and Can I Use websites. Other websites such as webstatus.dev use a bunch of information from web-features. The [W3C WebDX CG](https://www.w3.org/community/webdx/) (which currently maintains the repo) [recently discussed](https://github.com/web-platform-dx/web-features/issues/186) mapping web-features entries to browser standard positions too. This PR is a proposal for the first step towards achieving this. It adds IDs of features in the web-features repo to individual entries here. This is done by adding an optional `webFeaturesId` property to entries in `summary.json`. This property is a string which value matches filenames in https://github.com/web-platform-dx/web-features/tree/main/features (without the file extension). If you agree with this approach, this will help us link WebKit positions with a lot of other data about web platform features around various web properties. I opened a similar PR on Mozilla's positions repo, see https://github.com/mozilla/standards-positions/pull/1034 cc @annevk who we discussed this with.