Motivation
I'm thinking about how to make usethis less uv-centric. One issue is that adding dependencies is via uv inherently. Now, that's still quite good since it updates pyproject.toml and includes sensible lower bounds, etc. But we should be able to use --frozen so at the least the venv won't change. This will also allow the test suite to speed up significantly since many tests rely on the dependencies being declared, but not necessarily actually installing them (the slow part).

Summary of desired enhancement
Support a --frozen flag in the usethis tool <tool> --frozen interface. This would just be passed-down to uv subprocess calls.