tbl_custom_summary() default summary update

[ddsjoberg]

What changes are proposed in this pull request?
restoring feature from previous version of the package, where character statistics would have the default formatting function as.character() (thus no need to specify this for users, and this will no longer be a breaking change for v2.0).

FYI @larmarange

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Reviewer Checklist (if item does not apply, mark is as complete)

• Ensure all package dependencies are installed: renv::install()
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