add option in combine_checkboxes() to paste checkboxes when multiple selected

[d-morrison]

Description

Adds an option in combine_checkboxes() to concatenate checkbox values when multiple are selected.
Partially addresses #220; implements concatenation approach but not list-column approach.

Screenshots

Input:

> nonrepeat_data
# A tibble: 3 × 6
  study_id multi___1 multi___2 multi___3 single_checkbox___1 extra_data
     <dbl> <lgl>     <lgl>     <lgl>     <lgl>                    <dbl>
1        1 TRUE      FALSE     FALSE     TRUE                         1
2        2 TRUE      TRUE      FALSE     TRUE                         2
3        3 FALSE     FALSE     FALSE     FALSE                        3

Before:

# A tibble: 3 × 4
  study_id extra_data multi    single_checkbox
     <dbl>      <dbl> <fct>    <fct>          
1        1          1 Red      Green          
2        2          2 Multiple Green          
3        3          3 NA       NA      

After:

# A tibble: 3 × 4
  study_id extra_data multi       single_checkbox
     <dbl>      <dbl> <fct>       <fct>          
1        1          1 Red         Green          
2        2          2 Red, Yellow Green          
3        3          3 NA          NA        

PR Checklist

Before submitting this PR, please check and verify below that the submission meets the below criteria:

• New/revised functions have associated tests
• New/revised functions that update downstream outputs have associated static testing files (.RDS) updated under inst/testdata/create_test_data.R
  • not sure if needed?
• New/revised functions use appropriate naming conventions
• New/revised functions don't repeat code
• Code changes are less than 250 lines total
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Code Review

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Code Review Checklist

• I checked that new files follow naming conventions and are in the right place
• I checked that documentation is complete, clear, and without typos
• I added/edited comments to explain "why" not "how"
• I checked that all new variable and function names follow naming conventions
• I checked that new tests have been written for key business logic and/or bugs that this PR fixes
• I checked that new tests address important edge cases

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[d-morrison]

@rsh52 this PR partially addresses #220; glad to revise as helpful!