Update stats script

[ddbeck]

Towards #3510, this PR updates ./scripts/stats.ts to return a JSON object with counts of:

• The total features
• The total groups
• The total compat keys in our BCD dependency
• The unmapped compat keys
• The unmapped compat keys that are nomral (i.e., not nonstandard or deprecated in BCD)
• The unmapped compat keys that are discourageable (i.e., nonstandard or deprecated in BCD)
• The cumulative shipping days for each (i.e., for each requisite compat key, how many days has a browser shipped that feature in a stable release; see Generate new statistics #3510 (comment) for more details)
• The total caniuse IDs in our caniuse-lite dependency (excluding some dupes of BCD)
• The unmapped caniuseIds

The output looks like this:

{
  "featureCount": 1163,
  "groupCount": 103,
  "compatKeys": 17714,
  "unmappedCompatKeys": 2680,
  "unmappedCompatKeysNormal": 1691,
  "unmappedCompatKeysDiscourageable": 16023,
  "unmappedCompatKeysCumulativeShippingDays": 41875279,
  "unmappedCompatKeysCumulativeShippingDaysNormal": 11600644,
  "unmappedCompatKeysCumulativeShippingDaysDiscourageable": 30274635,
  "caniuseIds": 583,
  "unmappedCaniuseIds": 236
}

(Yes, some of those numbers are very large! This is expected.)

If you rerun it with --previous some-previous-run.json , it'll include a change key with the difference between the two runs.

Possible follow up work:

• Create a stats.json release artifact for comparing stats over time
• Create a regularly-posted comment thread similar to Feature statistics #788, possibly with graphs

[ddbeck]

This is wrong. It's summing all features, not just the discourageables.