The three.js preview's generic name made it unclear next to the VTK/Dual/
Multi/Batch previews. Renamed the node id (schema + both mappings), the
widget file (mesh_preview.js -> mesh_preview_three.js, extension name
geometrypack.meshpreview -> geometrypack.meshpreview.three), and the four
test workflows that reference the node type.
Breaking for external saved workflows that used GeomPackPreviewMesh (node
type no longer exists) -- accepted; the pack's own workflows are updated.
PozzettiAndrea