Add Media Session types

[Rowno]

This adds types for the Media Session API. Fixes microsoft/TypeScript#19473

The only thing that could be improved with the types is that:

interface MediaSession {
    setActionHandler(action: MediaSessionAction, handler: MediaSessionActionHandler | null): void;
}

interface MediaSessionActionHandler {
    (details: MediaSessionActionDetails): void;
}

should really be:

interface MediaSession {
    setActionHandler(action: 'seekbackward' | 'seekforward', handler: MediaSessionSeekActionHandler | null): void;
    setActionHandler(action: 'seekto', handler: MediaSessionSeekToActionHandler | null): void;
    setActionHandler(action: MediaSessionAction, handler: MediaSessionActionHandler | null): void;
}

interface MediaSessionActionHandler {
    (details: MediaSessionActionDetails): void;
}

interface MediaSessionSeekActionHandler {
    (details: MediaSessionSeekActionDetails): void;
}

interface MediaSessionSeekToActionHandler {
    (details: MediaSessionSeekToActionDetails): void;
}

But I couldn't figure out how to insert the additional interfaces and the MediaSessionSeekActionDetails and MediaSessionSeekToActionDetails types from the widl file.

[msftclas]

All CLA requirements met.

[saschanaz]

You might want to check addedTypes.json and its additional-signatures field.

[Rowno]

I spent a couple of hours hacking around in the addedTypes.json and overridingTypes.json files but couldn't figure out how to make it work properly.

[saschanaz]

We might need some documentation for those files... For now, something like this should work.

"callback-functions": {
  "callback-function": {
    "MediaSessionActionHandler": {
      "name": "MediaSessionActionHandler",
      "override-signatures": [
        "(details: MediaSessionSeekActionDetails): void"
      ]
    }
  }
}

And for additional-signatures:

"interfaces": {
  "interface": {
    "MediaSession": {
      "methods": {
        "method": {
          "setActionHandler": {
            "additional-signatures": [
              "setActionHandler(action: 'seekbackward' | 'seekforward', handler: MediaSessionSeekActionHandler | null): void"
            ]
          }
        }
      }
    }
  }
}

[sandersn]

Additionally, caniuse.com says that only Chrome (and new Edge) support MediaSession API, so I want to wait until another browser supports this API before putting it in the standard library.

[sandersn]

Should wait until more than one engine implements it.

[Rowno]

Fair enough

[sapphi-red]

It seems MediaSession API has landed Firefox in version 82.

[saschanaz]

This is shipped by #1034

@github-actions close

[github-actions]

Closing because @saschanaz is a code-owner of all the changes.