Add crash report storage feature

[ddbeck]

Fixes #3468

[captainbrosset]

I think this API builds on top of the Reporting API, which should likely be named here.

[captainbrosset]

After re-reading a few resources, my understanding is now that:

1. The Reporting API is the higher-level feature, which basically consists in defining a Reporting-Endpoints header to say where the browser should send (multiple types of) reports to.
   That's https://web-platform-dx.github.io/web-features-explorer/features/reporting/

2. On top of this, the Crash Reporting API was defined. It's an extension of the Reporting API that's specifically made for reporting crashes, and which defines that reports contain data such as reason, stack, is_top_level, visibility_state, and crash_report_api.
   Spec: https://wicg.github.io/crash-reporting/
   We don't have a feature in the repo for this.

3. The window.crashReport object, which seems to be and extension of the Crash Reporting API that lets developers also send their own custom data along with a crash report, but creating a map of key/value pairs
   Spec: https://wicg.github.io/crash-reporting/#crash-report-storage
   That's the new crash-report-storage feature you're proposing here.

I'm not 100% confident that 2 and 3 are separate things, but they do seem this way to me.

That said, I don't know if there are BCD keys for 2.

[captainbrosset]

If there are no practical benefits to creating a feature for 2 and 3, then we should at least revise the description to broaden the language so it encompasses the Crash Reporting API too.