Add github action to update snapshot tests in PRs

in #79, we saw that making innocuous changes will cause snapshot tests to fail. This is expected and necessary, but I had to intervene and run and accept the snapshot tests from my own machine to get them to work.

I think it would be neat to have an action that would run and accept the snapshot tests from a github comment (which may exist already).

Start by reading issue #79 and reviewing the repository's existing snapshot-test and GitHub Actions setup. The work is complete when a pull request comment can trigger the snapshot tests and accept the expected updates without requiring a contributor to run them locally.

Written by the indexing model from the issue text.