Support SPARQL-style PREFIX declarations

Amazing tool!

When I type this into VS Code:

```
PREFIX rdfs
```

and hit `<TAB>`, I get:

```
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
```

I would prefer to use the SPARQL syntax here, so it would be nice if the LSP preserved that. The SPARQL syntax is the preferred one for RDF 1.2 and Jena has switched to it as the default, even for RDF 1.1 files.