Support MoBi projects in exportProjectToSnapshot()

[PavelBal]

exportProjectToSnapshot() (introduced when splitting the deprecated convertSnapshot()) currently converts PK-Sim project files (.pksim5) into snapshots (.json) only.

Extend it to also export MoBi projects to snapshots.

Tasks

• Add support for MoBi project -> snapshot conversion
• Detect/route by project type (PK-Sim vs MoBi)
• Tests with a MoBi project fixture
• Update docs (R/utilities-snapshots.R Roxygen + snapshots vignette) to drop the "PK-Sim only" caveat once supported

See also the sibling issue for loadProjectFromSnapshot() (#1973).