Deduplicate MIME-type-to-file-extension mapping in sparql-qlever

[ddeboer]

`@lde/sparql-qlever`'s importer maintains its own `supportedFormats` map (`application/n-triples` → `nt`, etc.) and `fileFormatFromMimeType()` function. `@lde/dataset` already exports `rdfFormatToFileExtension()` doing the same mapping.

Unify by reusing the existing utility from `@lde/dataset` instead of maintaining a parallel lookup.