Add prefix: sstim

[github-actions]

Closes #2055

[ttm]

Thanks for the automated PR. Two things from my side as the submitter, neither blocking.

The comment field picked up the whole Additional Comments text from the issue, including the parts addressed to you rather than to future readers of the record. Neighbouring records use a single line, so here is a trimmed value if it saves you the edit:

Content negotiated at https://w3id.org/sstim: Turtle, JSON-LD and RDF/XML by Accept header, while an HTML request returns a namespace page. Releases are frozen, snapshotted and archived under a concept DOI.

And the one open question, repeated here so it does not stay buried in that field. SSTIM is modular, and three sibling term namespaces exist under the same authority, all with hash IRIs and the same local-name pattern: https://w3id.org/sstim/vocab# (405 local names, the SKOS controlled vocabulary), https://w3id.org/sstim/exposure# (268) and https://w3id.org/sstim/ecosystem# (82), against 344 in the core namespace requested here. I am happy to file sstim.vocab and siblings as separate prefixes, or to switch the primary uri_format, whichever fits the Bioregistry model better. The core namespace was requested because it is what prefix.cc already serves for sstim.