Export class names used in theme so applications can use them

As discussed [here](https://github.com/MetaCell/meta-diagram/pull/45#discussion_r1212970408) we should export the class names used in the default meta-diagram theme to allow applications (like psyneulink) to use those (instead of having to hardcode those names in the application)

Start with the discussion in pull request #45 and trace where the default meta-diagram theme defines its class names. Identify the public entry point for exporting those names, then verify that an application such as psyneulink can consume them instead of hardcoding the values.

Written by the indexing model from the issue text.