Adding modify_source_note() function

[ddsjoberg]

What changes are proposed in this pull request?

• Added functions modify_source_note() and remove_source_note() for adding and removing source notes. (Support for multiple source notes within gtsummary #1910)

If there is an GitHub issue associated with this pull request, please provide link.
closes #1910

A few more details: Source notes have been a part of gtsummary for a long time, but there has never been a user-facing interface for adding them. In this PR, we add functions to add and remove source notes from a table. The way that source notes are saved has been updated to be more general (that is, so we can add more than a single source note). There may have been some advanced users who accessed the internals and added source notes, and if they had, this will be a breaking change.

---

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".

[ayogasekaram]

Looks great to me! Thank you @ddsjoberg