2130 gut modify table styling

[ddsjoberg]

What changes are proposed in this pull request?

• Added new function remove_column_merge() to undo a column merge. (Gut any remaining "work" being done in modify_table_styling() and migrate the code to individual functions (I think this only applies to column merging). #2130)

Also removed calculations from modify_table_styling() to the individual modify functions where possible.

If there is an GitHub issue associated with this pull request, please provide link.
closes #2130

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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".