add missing checkbox export

[salimkanoun]

Context

The checkbox component was not exported at the root of the ui package

Changes & Results

Testing

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] OS:
• [] Node version:
• [] Browser:

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[codecov]

Codecov Report

Merging #3629 (bb5a1a1) into master (9045dda) will not change coverage.
Report is 2 commits behind head on master.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3629   +/-   ##
=======================================
  Coverage   41.94%   41.94%           
=======================================
  Files          79       79           
  Lines        1421     1421           
  Branches      341      341           
=======================================
  Hits          596      596           
  Misses        658      658           
  Partials      167      167           

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