/art, /rush-duel art: reduce calls to Yugipedia

[kevinlul]

/art no longer calls Yugipedia by assuming the Master Duel redirect exists if there is Master Duel rarity data.
/rush-duel art reduces calls to Yugipedia by skipping the redirect check when illustrations are already specified.
Rework some illustration URL construction logic to create a common yugipediaFileRedirect helper, with unit testing.
Add some unit tests for the ArtSwitcher.

Closes #443

[codecov]

Codecov Report

Attention: Patch coverage is 59.61538% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 45.84%. Comparing base (d87c07b) to head (65b6bff).

Files	Patch %	Lines
src/commands/rush-duel.ts	40.74%	16 Missing ⚠️
src/commands/art.ts	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
+ Coverage   44.29%   45.84%   +1.55%     
==========================================
  Files          33       32       -1     
  Lines        4569     4541      -28     
  Branches      186      203      +17     
==========================================
+ Hits         2024     2082      +58     
+ Misses       2544     2459      -85     
+ Partials        1        0       -1     

Flag	Coverage Δ
contract	?
unit	45.84% <59.61%> (+2.54%)	⬆️

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