add support for atom_site.pdbx_sifts_xref export

data/cif-field-names/mmcif-field-names.csv

src/mol-io/reader/cif/schema/mmcif.ts

src/mol-model-props/sequence/sifts-mapping.ts

src/mol-model-props/sequence/themes/best-database-mapping.ts

src/mol-model/structure/export/categories/atom_site.ts

...odel/structure/structure/util/superposition-db-mapping.ts

src/mol-plugin-ui/structure/superposition.tsx

...or/dynamic/custom-props/sequence/best-database-mapping.ts