Adding additional STEP entity types

[jmacentee]

I've been working on an app to convert STEP to SVG for laser cutting. From time to time, I've bumped into sample step files that couldn't be parsed with your library. I'm using Claude Opus to help me with the hard parts that are over my head. This is what it created to get past my problems. Please feel free to accept this or cherry pick from it if you find it useful. For the latest change, I had it add an md file explaining why the changes were needed.

[brettfo]

This repo isn't under active development and I'm unlikely to get around to reviving it any time soon. Ultimately the hard-coded parsing approach isn't the correct way to handle STEP files because they're defined by other schema files and I just haven't had the time to fully investigate that approach. I've started that method in the schema-parser branch but that is also unlikely to see any attention in the near future.