Change plot_label to use geom_text and add support for selecting which facet to plot on

[aaronw22]

[TODO] Update the plot labelling vignette because we don't need the hacky thing for facets anymore.

Describe your changes

Do the following before requesting a review

• [ ] I have written tests covering functions I have added or changed.
• [ ] I have run tests and they all pass.
• [ ] I have ensured all new functions will show up in the _pkgdown.yml file.
• [ ] I have updated DESCRIPTION with any new package dependencies and the new package version number.
• [ ] I have updated NEWS.md with a brief description of my changes.
• [ ] I have updated the package documentation with devtools::document().
• [ ] I have run devtools::build_readme() to update README.md.
• [ ] I have updated the package website with pkgdown::build_site_github_pages().

Post-release

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