Move default branch from `master` to `main`

[njtierney]

The default branch of this repository is being moved from master to main. This issue will be closed when the default branch has been changed.

For help and guidance with updating the default branch of your local or forked copy of this repo, please see Renaming the default branch.

Search for files that may potentially reference master.

^{ID: 91e77a361e4e}

[njtierney]

The default branch has been renamed to main. If you need to update a local copy of this repo, you can use the usethis package, v2.1.2 or higher:

usethis::git_default_branch_rediscover()