Use @rdfjs/types for RDF types instead of rdf-js

[surilindur]

Using jest-rdf in a project that does not include rdf-js itself causes TypeScript compiler errors:

node_modules/jest-rdf/index.d.ts:1:22 - error TS2307: Cannot find module 'rdf-js' or its corresponding type declarations.

1 import * as RDF from "rdf-js";
                       ~~~~~~~~

This should fix that.

[coveralls]

Pull Request Test Coverage Report for Build 7489994402

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.851%

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Totals
Change from base Build 6622646142:	0.0%
Covered Lines:	130
Relevant Lines:	131

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💛 - Coveralls

[rubensworks]

Thanks! Released as 1.8.1.