This function adds files/folders to the .Rbuildignore file. If a
.Rbuildignore is already present, files to be ignored while checking
package are just added to this file. Otherwise a new file is created.
See also
Other development functions:
add_dependabot(),
add_dependencies(),
add_github_action(),
add_issue_template(),
add_r_depend(),
add_to_gitignore()
