#4500: Add support for checkboxes in `@pixiebrix/forms/data`

[fregante]

What does this PR do?

• Fixes https://github.com/pixiebrix/pixiebrix-extension/issues/4500

Future Work

• [ ] Would be good to use https://github.com/jefersondaniel/dom-form-serializer instead

Checklist

• [x] Add tests
• [ ] Run Storybook and manually confirm that all stories are working
• [x] Designate a primary reviewer: @twschiller

[codecov[bot]]

Codecov Report

Merging #4513 (712d295) into main (2fb7c56) will increase coverage by 0.06%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main    #4513      +/-   ##
==========================================
+ Coverage   50.41%   50.48%   +0.06%     
==========================================
  Files         905      905              
  Lines       26820    26828       +8     
  Branches     5441     5442       +1     
==========================================
+ Hits        13522    13543      +21     
+ Misses      12371    12359      -12     
+ Partials      927      926       -1     

Impacted Files	Coverage Δ
src/blocks/transformers/FormData.ts	86.95% <90.90%> (+86.95%)	:arrow_up:
src/helpers.ts	44.00% <0.00%> (+4.00%)	:arrow_up:

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[github-actions[bot]]

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