Option to restrict editor so it only allows one molecule to be drawn

**Background**
There are use cases when a user's ability to draw more than one molecule in a given editor, is not ideal.

**Solution**
Have a prop (eg "singleMolecule: boolean) that is passed and makes it so the user can only draw one molecule in the editor

The issue names no file, component, or test. Start by locating the editor's public props and the logic that permits multiple molecules, then identify the relevant test or usage surface. The work is done when the option prevents drawing a second molecule while the default behavior remains unchanged.

Written by the indexing model from the issue text.