Write readable indentation to VCS-friendly solution source files

Make source files in VCS-friendly .objosln solutions as readable in ordinary Git diffs as they are in Objo Studio. Each .objobasic file should contain leading spaces that match Studio's calculated visual indentation.

This indentation is an on-disk projection for people and Git tooling only. Studio must remove the generated indentation while loading, keep its existing canonical source model and editor documents unindented, and continue to render indentation visually in the editor exactly once. Compilation and runtime behaviour must not change.