5594 bold distractor letter

[TylerZeroMaster]

part of openstax/cnx-recipes#5594

Books included in change (maybe more, but at least these based on the updates to expected output):

• data-science
• information-systems
• lifespan-development
• marketing
• nursing-external
• nursing-internal
• python

This change alone should not cause any visible differences. Instead, the addition of a span wrapper is meant to facilitate uniform styling of the letter answers. I named the class answer-letters instead of letter-answers because there already existed a problem-letter class and answer-letters seemed to compliment this nicely.

I also added a multi-choice question to multipart question tests because I did not see any included. This helped me verify that letter answers are not added to multi-choice answers contained inside multipart questions (which is probably good because it might be confusing to see something like a. d. The answer to part a is letter d).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (c9571a6) to head (b74ed39). Report is 6 commits behind head on main.

@@            Coverage Diff            @@
##              main      #395   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          215       215           
  Lines         3559      3561    +2     
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+ Hits          3559      3561    +2     

Files with missing lines	Coverage Δ
...jected_exercise/bake_injected_exercise_question.rb	100.00% <100.00%> (ø)