Improvements to the report format

[foolip]

Braindump at the request of @jugglinmike and @lolaodelola. @ddbeck @Elchi3 @queengooborg FYI.

This is the structure of mdn-bcd-collector reports currently:

{
    "__version": "6.2.7",
    "results": {
        "https://mdn-bcd-collector.appspot.com/tests/": [
            {
                "exposure": "ServiceWorker",
                "name": "api.AbortController.AbortController",
                "result": true
            },
            {
                "exposure": "SharedWorker",
                "name": "api.AbortController.AbortController",
                "result": false
            },
            {
                "exposure": "Window",
                "name": "api.AbortController.AbortController",
                "result": true
            },
            {
                "exposure": "Worker",
                "name": "api.AbortController.AbortController",
                "result": true
            },
            {
                "exposure": "Window",
                "message": "threw TypeError: Failed to construct 'Notification': Illegal constructor. Use ServiceWorkerRegistration.showNotification() instead.",
                "name": "api.Notification.actions",
                "result": null
            },
            {
                "exposure": "Worker",
                "message": "threw TypeError: Failed to construct 'Notification': Illegal constructor. Use ServiceWorkerRegistration.showNotification() instead.",
                "name": "api.Notification.actions",
                "result": null
            }
        ]
    },
    "userAgent": "Mozilla/5.0 (Linux; Android 10; SM-G960U) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Mobile Safari/537.36"
}

A few things that could be improved:

• https://mdn-bcd-collector.appspot.com/tests/ as a key isn't really needed. Originally it was to test using both HTTP and HTTPS and combine results, but that's long time. The update-bcd script only uses the Object.values(report.results), not the keys.
• Nothing allows mapping results for api.Document.mozCancelFullScreen to api.Document.exitFullscreen will an alternative_name. Handle prefixes and alternative names #431 remains unsolved.
• There can be multiple results for a single path, leaving it to update-bcd to combine them. To allow moving update-bcd to BCD itself and consuming reports without much knowledge of the collector, it might be better to combine/map results when producing reports, not when consuming them.
• The reports could probably be more compact if represented as a tree, more like BCD.