updating the core gtsummary code file

[ddsjoberg]

What changes are proposed in this pull request?
Updated the gtsummary core code script to refer to all functions with the pkg:: so other packages that copy the file don't need to import the same functions as gtsummary in the NAMESPACE. Now they just need to depend on the same packages.

Also added some other core code that help with selecting variables to the core code script.

If there is an GitHub issue associated with this pull request, please provide link.
This closes #454

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Checklist for PR reviewer

• PR branch has pulled the most recent updates from master branch
• NEWS.md has been updated under the heading "# gtsummary (development version)"?
• usethis::use_spell_check() runs with no spelling errors in documentation
• If a new function was added, function included in pkgdown.yml
• If a bug was fixed, a unit test was added for the bug check
• Run pkgdown::build_site(). Check the R console for errors, and review the rendered website.
• Code coverage is suitable for any new functions/features.
• R CMD Check runs without errors, warnings, and notes
• When the branch is ready to be merged into master, increment the version number using usethis::use_version(which = "dev"), run codemetar::write_codemeta(), approve, and merge the PR.