fix: web-ext should look for the extension id in browser_specific_settings and fallback to applications

[rpl]

Fixes #1901

This PR includes and supersedes #1944

• the first patch contains a squashed version of feat: Add support for browser_specific_settings #1944 as submitted by the author (I did preserve Lusito as the author of that patch because it did change enough from the PR it was originally based on),
• the second patch applies some small tweaks to:
  • better match Firefox behavior (and some additional test case to cover the expected behaviors)
  • fix some new flow typecheck errors (due to the updated flow version)
  • small tweaks suggested in the last round of review comments

This PR also enable the optional chaining support in the flow config file.

[coveralls]

Coverage remained the same at 100.0% when pulling fd4916e on rpl:fix/browser-specific-support into a6eb2a9 on mozilla:master.