feat: expose modeler.designs attribute

[RobPasMue]

Description

Allow retrieval of open designs easily through public attribute.

Issue linked

Closes #1400

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate unit tests.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have added the minimum version decorator to any new backend method implemented.
• I have made sure that the title of my PR follows Conventional commits style (e.g. feat: extrude circle to cylinder)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.79%. Comparing base (18e5478) to head (7598ada).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1401   +/-   ##
=======================================
  Coverage   91.79%   91.79%           
=======================================
  Files          86       86           
  Lines        6821     6824    +3     
=======================================
+ Hits         6261     6264    +3     
  Misses        560      560           

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