Point mutation documenation/function

[hfuchs5]

What changes are proposed in this pull request? A new exported helper function which allows users to identify and rename specific point mutations.

If there is an GitHub issue associated with this pull request, please provide link. #322

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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. Ensure the pull request branch and your local version match and both have the latest updates from the main branch.
• [ ] 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 withr::with_envvar(new = c("NOT_CRAN" = "true"), covr::report()). 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 "# cbioportalR (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).
• [ ] Run codemetar::write_codemeta()
• [ ] Run usethis::use_spell_check() again
• [ ] Approve Pull Request
• [ ] Merge the PR