Fix: save_babylonjs_to_file

[pgibertini]

• Please check if the PR fulfills these requirements

• [x] The commit messages follows our guidelines (explicit, in english)

• [ ] Tests for the changes have been added (for bug fixes / features)

• [x] Docs have been added / updated (for bug fixes / features)

• [x] CHANGELOG.md has been updated

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• [x] Bug fix: Add missing dark_mode parameter in save_babylonjs_to_file method.

• [ ] New features:

• [ ] Documentation

• [ ] Optimization

• Other information: