fix: Report a warning on manifest_version 3 extensions targeting Firefox for Android

[rpl]

Fixes #5104

TODO List

• replace the placeholder for the shortened url to a more detailed doc page with a real moz.la link (moved to separate followup: Add link to additional docs related to limitations in the MV3 implementation on Firefox for Android #5110)

┆Issue is synchronized with this Jira Task

[willdurand]

Looks good to me overall.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (50380ff) 98.73% compared to head (eecd911) 98.73%.
Report is 14 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5105   +/-   ##
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  Coverage   98.73%   98.73%           
=======================================
  Files          54       54           
  Lines        2922     2927    +5     
  Branches      888      891    +3     
=======================================
+ Hits         2885     2890    +5     
  Misses         37       37           

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[rpl]

@willdurand as we agreed earlier today, I've filed a separate followup to add the shorted link in the description of the new warning (tracked by #5110), then updated this PR accordingly. This should be ready for your final review pass.