Undocumented prefixes used for local storage

Background

According to https://eyeo.gitlab.io/adblockplus/abc/webext-sdk/master/docs/#shared-resources, EWE only stores keys with the "file:" prefix using browser.storage.local. However, after #231 (closed), this is no longer accurate, because it no longer uses "file:" as prefix but "abp:file:".

Additionally, it started using "abp:pref:" to replace "pref:", which wasn't mentioned in the documentation yet.

Use case

Consumers rely on well-documented usage of shared resources, in order to avoid naming conflicts.

What to change

• In https://eyeo.gitlab.io/adblockplus/abc/webext-sdk/master/docs/#shared-resources:
  • Mention EWE's usage of browser.storage.local
  • Mention EWE's usage of browser.storage.session
  • Mention other messages which start with the ewe: prefix (it's not just ewe:content-hello)
• (optional for now) Replace usage of "abp:" prefix with "ewe:" for browser.storage.local, for consistency across all shared resources, and to further avoid potential naming conflicts with Adblock Plus-specific preferences.