Allowing multiple spanning headers with modify_spanning_header(level)

[ddsjoberg]

What changes are proposed in this pull request?

• The modify_spanning_header(level) argument has been added to allow for multiple levels of spanning headers in the resulting tables. The remove_spanning_header() function has also been added to ease the removal of spanning headers. (Spanning header updates #2099)

If there is an GitHub issue associated with this pull request, please provide link.
closes #2099

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

• PR branch has pulled the most recent updates from main branch.
• 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
• All GitHub Action workflows pass with a ✅

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 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".