New metric to detect service workers in pwa.js

[demianrenzulli]

This PR contains the following changes to pwa.js:

• Added a new field serviceWorkerInfo that uses a new regular expression to detect service worker registrations, that matches the cases that the original one was ignoring: for example relative paths and variables used as the service worker URL. Here is an example test for mobile.twtiter.com, which was originally ignored and now it's found under serviceWorkerInfo.
• Added the new variable serviceWorkerInfo to the conditions for the serviceWorkerHeuristic. Ideally, the number of detected service workers should approximate more closely to serviceWorkerHeuristic.
• Added the fetch event listener to swEventListenersInfo. We initially removed the "fetch" keyword to avoid false positives (since the event can be used in pages too), but we recently realized that the pattern: addEventListener("fetch", ...), can only happen inside a service worker, so it makes sense to add it, at least there.

We should discuss if the original serviceWorkers continues making sense or not. Maybe after seeing the data, we'll have a better idea.

Unfortunately, these changes won't make it for the 2021 version of Web Almanac, but we thought about leaving them prepared for 2021.

cc // @tunetheweb @rviscomi

[rviscomi]

I've deployed this metric to the HA server so we should have ~95% coverage in the August dataset.