Fix #964 Add types for checkboxes

[seratch]

Summary

This pull request fixes #964 by adding a type for the checkboxes type block element. The radion_buttons one already exists.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #966 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
+ Coverage    94.8%   94.87%   +0.06%     
==========================================
  Files          12       12              
  Lines         732      741       +9     
  Branches      166      166              
==========================================
+ Hits          694      703       +9     
  Misses         13       13              
  Partials       25       25

Flag	Coverage Δ
#eventsapi	89.61% <ø> (-10.39%)	⬇️
#interactivemessages	94.92% <ø> (-5.08%)	⬇️
#webapi	97.88% <ø> (ø)	⬆️
#webhook	87.5% <ø> (+1.38%)	⬆️

Impacted Files	Coverage Δ
src/index.ts	87.5% <0%> (ø)	⬆️
src/http-handler.ts	90.57% <0%> (ø)	⬆️
src/util.ts	89.47% <0%> (ø)	⬆️
src/errors.ts	100% <0%> (ø)	⬆️
src/adapter.ts	95.14% <0%> (ø)	⬆️

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