Get WICG entries from browser-specs (#847)

As alluded to in #812, now that WICG entries are no longer pulled from the
(unmaintained) biblio file in the WICG repo, updates to existing WICG entries
require manual intervention.

This update drops all WICG entries that browser-specs can contribute so that
further updates to these entries can be handled automatically, and updates
`browser-specs.json` accordingly (result of running the script once WICG
entries have been removed).

Remaining WICG entries are:
- retired specs that browser-specs does not (and will not) have.
- `WICG-` prefixed entries, all turned into aliases.
- a couple of aliases for specs that are known under a different name in
browser-specs.