fix: add Monaco to 3S (EU) list

[annuh]

Add Monaco to the 3S (EU) list. 🇲🇨

This will fix the following error when sending something to Monaco: Countries in receiver and sender address not allowed together for this product

Source: https://www.postnl.nl/tarieven/zone-indeling/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.11%. Comparing base (98949ae) to head (7bc7ef1).
⚠️ Report is 5 commits behind head on v2.0.x.

Additional details and impacted files

@@            Coverage Diff            @@
##             v2.0.x     #103   +/-   ##
=========================================
  Coverage     69.11%   69.11%           
  Complexity     2214     2214           
=========================================
  Files           124      124           
  Lines          6446     6446           
=========================================
  Hits           4455     4455           
  Misses         1991     1991           

Files with missing lines	Coverage Δ
src/PostNL.php	69.39% <ø> (ø)

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[firstred]

Thanks for the PR!