moved cellpack to toplevel extensions dir and register as behavior

src/apps/viewer/extensions/jolecule.ts

src/apps/viewer/index.ts

src/extensions/cellpack/color.ts

src/extensions/cellpack/curve.ts

src/extensions/cellpack/data.ts

src/extensions/cellpack/index.ts

src/extensions/cellpack/model.ts

src/extensions/cellpack/preset.ts

src/extensions/cellpack/property.ts

src/extensions/cellpack/state.ts

src/extensions/cellpack/util.ts