Support `definition` aka Go To Definition

[multimeric]

This is the first thing I looked for in swls, but it doesn't seem to be supported. Basically I want to be able to Ctrl+Click on an IRI and for the editor to jump to its definition. My use case is for JSON-LD, but it should be doable for other serialization formats too.

For example, if you have:

{
    "@graph": [
        {
             "@id": "https://foo.com",
             "somePredicate": "foo"
        },
        {
            "@id": "https://bar.com",
            "otherPredicate": {"@id": "https://foo.com"}
        }
    ]
}

Then you click on the {"@id": "https://foo.com"} on line 9, it should jump you to line 4 or so, where the entity is defined.

This is the relevant part of the LSP spec. I'm happy to write this feature if a maintainer expresses interest in it being merged.