Proposal to map focus areas to web-features

[captainbrosset]

Fixes #928.

This PR maps the Interop focus areas, per year, to web-features IDs.

The mapping lives in a single file, at the root of the directory. This way, consumers of this data can get the entire data at once, for all years.

The object is structured as follows:

{
  <interop-year>: {
    <interop-dashboard-focus-area-identifier>: [<web-features-ID>]
  }
}

<interop-dashboard-focus-area-identifier> are the unique IDs that the interop dashboard uses to identify focus areas in the graph drop-down. These IDs are unique for a given year. They may repeat across years, which is fine because the object is key-ed by interop year anyway.

Based on this data, a consumer can retrieve the list of interop year, and for each, the list of focus areas, and then for each of these, the list of web-features that they map to.

Based on this, a consumer could, in theory, also retrieve the WPT test labels for each focus area (although that would require getting the interop-data from the wpt.fyi repo too). But I think the main use case for consumers would be to link to the interop dashboard, ideally at the right place, showing the right graph (see web-platform-tests/wpt.fyi#4325 about this).

[captainbrosset]

@foolip I've added you as a contributor to my fork of this repo so you can push commits to my PR branch.

[captainbrosset]

@foolip If you can, please take a look. I've updated the description of the PR with more details, and added many more mappings.
I'm not done, but the mappings that are left are much harder for me to find. A lot of them are "webcompat", or other similar areas which seem to contain mostly unrelated bug-fixing-type tests that don't necessarily correspond to web-features.

[foolip]

Looks good, some things need fixing before landing. Thank you!