Modify header updates

[ddsjoberg]

What changes are proposed in this pull request?

1. The internals of modify_header() have been updated so users may more easily access internal data while defining headers. For example, whenever a user updates the header of tbl_summary(by=) column, they will have access to the level, and the Ns within that level. Previously, they needed to use a stat_by= argument that did not fit well with the structure of a general gtsummary object. There is no longer a need to call this function: the users will simply have access to these values via teh glue syntax when defining the header. Moreover, these counts are now available for tbl_svysummary() objects including the unweighted counts.
2. Internally, the modify_header_internal() function has been removed and simply replaced with modify_header().
3. The modify_header() function no longer adds its call to the gtsummary .$call_list. The call list is used to more easily re-construct the series of gtsummary calls if needed. However, the column headers can easily be re-constructed from the .$table_header object.

closes #668

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Checklist for PR reviewer

• PR branch has pulled the most recent updates from master branch. Ensure the pull request branch and your local version match and both have the latest updates from the master branch.
• If an update was made to tbl_summary(), was the same change implemented for tbl_svysummary()?
• 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. Review coverage with covr::report(). Before you run, set Sys.setenv(NOT_CRAN="true") and begin in a fresh R session without any packages loaded.
• R CMD Check runs without errors, warnings, and notes
• usethis::use_spell_check() runs with no spelling errors in documentation

When the branch is ready to be merged into master:

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