Stealth extension

[vringar]

This PR is a rebased version of #1036 to support the merged extension.

Iirc we discussed upstreaming this with @bkrumnow already and the GPLv3 should allow us to merge this back with any issues. Using MPL licensed code is GPLv3 is also no issue.

closes #1031
closes #448
closes #1081

[bkrumnow]

@vringar Let me know, if there is anything needed. I am happy to help.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f72e7ca) 45.73% compared to head (0cfc12f) 45.74%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
+ Coverage   45.73%   45.74%   +0.01%     
==========================================
  Files          35       35              
  Lines        3481     3482       +1     
==========================================
+ Hits         1592     1593       +1     
  Misses       1889     1889              

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[vringar]

@bkrumnow as this is finally building, iirc you had you had a detection website for the current version of the JSInstrument.
Could you share that again, so I can turn it into tests that verify that the stealth extension can't be detected.

My next step would be to replace the current JS Instrument with your implementation as it seems strictly better (are there any limitations that I missed?), so this will get merged once the stealth implementation passes the JSInstrument tests and there are tests that verify that stealth still works.