Motivation
It's good to have an interface to add a license to a project (including switching licenses).

Since most VCS hosting platforms like GitHub already have a nice GUI for initializing a new repo with a license, this is low priority.

Summary of desired feature
Provide a usethis license implementation including a --switch flag to change the existing license.
Note that it is possible to have multiple licenses (e.g. uv's dual-license approach). This might be sufficiently uncommon as to not be necessary, but some thought should be given when designing this feature.