Adding 'card' print method

[ddsjoberg]

What changes are proposed in this pull request?
Adding print.card() method.

@bzkrouse what do you think of the print method? Once we finalize, I'll finish the PR which will lead to some changes in snapshot testing I think.

What the print method does:

• converts object to data frame so the list columns show the values (unlike a tibble print)
• prints between 10 and 20 rows
• messaging for the number of rows/columns that are not shown
• Removes warning and error columns if nothing to report
• if there are more than 6 columns, statistic_fmt_fn/contex columns removed
• A max of 9 characters are displayed for columns 'group##_level', 'variable_level', 'stat_label', and 'context'
• If a statistic is a double, then it is rounded to 3 decimal places
• If a proper function is found in the statistic_fmt_fn column, it's printed as "<fn>" to save space

Reference GitHub issue associated with pull request. e.g., 'closes #1'
closes #93

---

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

• Ensure all package dependencies are installed: devtools::install_dev_deps()
• PR branch has pulled the most recent updates from master branch: usethis::pr_merge_main()
• If a bug was fixed, a unit test was added.
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features: devtools::test_coverage()
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged:

• Update NEWS.md with the changes from this pull request under the heading "# cards (development version)". If there is an issue associated with the pull request, reference it in parentheses at the end update (see NEWS.md for examples).
• Increment the version number using usethis::use_version(which = "dev")
• Run usethis::use_spell_check() again
• Approve Pull Request
• Merge the PR. Please use "Squash and merge".

[ddsjoberg]

I think this one is ready for review now, @bzkrouse ! Thanks!

[bzkrouse]

This is really nice!!

[bzkrouse]

I think the readme needs re-rendering after the last update :)

[ddsjoberg]

ahhh good catch! thank you!