CompatHelper: add new compat entry for "MeshCat" at version "0.9"

[github-actions[bot]]

This pull request sets the compat entry for the MeshCat package to 0.9.

This is a brand new compat entry. Previously, you did not have a compat entry for the MeshCat package.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.